Retirement Plan Eligibility: Age, Service, and Entry Dates

Under federal law, the latest an employer can make you wait to join a 401(k) plan is until you turn 21 and finish one year of service, and many employers let you in sooner. That is the ceiling set by the Employee Retirement Income Security Act, and the 401(k) eligibility rules that follow from it govern when you can start contributing, who can be left out, and how recent changes have opened the door for part-time workers.¹

The Age 21 and One Year of Service Limit

No 401(k) plan can require you to be older than 21 or to have worked more than one year before you can participate. That is a maximum, not a minimum. Plenty of employers open the plan on your hire date, after 30 days, or after 90 days, because faster access helps recruiting. Plans also cannot impose a maximum age, so an older new hire cannot be shut out for that reason.¹

There is one legal exception to the one-year cap. A plan that vests you 100% in all employer contributions right away can require up to two years of service before you join. You rarely see this, but it is allowed.¹

What a “Year of Service” Means

A year of service is a 12-month period in which you work at least 1,000 hours. The clock usually starts on your hire date. If you reach 1,000 hours by your first anniversary, you are credited with a year of service. If you fall short in that first 12 months, the plan can switch to measuring on the plan year instead.¹ For a standard full-time employee the math takes care of itself. If your schedule hovers near the line, track your hours.

When You Actually Start Contributing

Meeting the age and service requirements does not always mean you start deferring from your next paycheck. Most plans designate specific entry dates to keep enrollment orderly. Common choices are the first of the next month, the start of the next quarter, or semi-annual dates like January 1 and July 1.

Federal law caps that administrative delay. You must enter the plan no later than the earlier of six months after you qualified or the first day of the next plan year after you qualified.¹ If you hit the eligibility rules on March 10 and the plan uses quarterly entry, you join on the next quarterly date; what the plan cannot do is push you to the following January.

Your employer must give you a Summary Plan Description that spells out eligibility and entry dates for the specific plan.² If you have not seen one, ask HR. It is the authoritative answer for your situation.

Part-Time Workers and the Long-Term Part-Time Rule

Before 2024, a part-time employee who never cleared 1,000 hours in a year could work the same job for decades without ever qualifying for the 401(k). The SECURE Act and SECURE 2.0 closed that gap. Employers now have to allow participation for workers who log at least 500 hours per year over two consecutive 12-month periods.³ The original rule required three consecutive years; SECURE 2.0 shortened that to two years beginning with the 2025 plan year.

Once you qualify under this rule, you can make your own salary deferrals. Employers are not required to make matching or other employer contributions for long-term part-time participants.⁴ A part-time worker who gets in this way can save their own money tax-deferred but should not assume the match available to full-timers applies. Check the plan document.

Collectively bargained employees and nonresident aliens with no U.S.-source income are outside this expansion, so the new access mainly reaches non-union, domestic part-time staff.

Automatic Enrollment in Newer Plans

If your 401(k) or 403(b) plan was set up after December 29, 2022, automatic enrollment applies starting with the 2025 plan year. Your employer will begin deducting contributions from your paycheck unless you affirmatively opt out.⁵

The default rate must be at least 3% of pay but no more than 10%. Each year the rate steps up by one percentage point until it reaches at least 10%, with a ceiling of 15%.⁵ You can change the rate or stop contributing at any time.

The automatic enrollment mandate does not reach every employer. Businesses that have existed fewer than three years are exempt, as are employers with 10 or fewer employees. Church and government plans are exempt regardless of size. Plans that were already in place on December 29, 2022, are grandfathered, though many add automatic enrollment anyway because it lifts participation.⁵

Workers Who Can Be Excluded

Even if you clear the age and service bars, certain categories of workers can be left out of a plan.

Union employees covered by a collective bargaining agreement are commonly excluded when retirement benefits were part of the bargaining. The theory is that those workers negotiate their own retirement arrangements through the contract.

Nonresident aliens with no U.S.-source income can be excluded as well.⁶

Independent contractors are not eligible because they are not employees. The IRS looks at the actual working relationship, not the label on the paperwork, to decide which you are.⁷ If you have been classified as a contractor but function as an employee, misclassification can be a basis for later arguing you should have been in the plan.

Highly compensated employees are not excluded, but their contributions can be capped. For the 2026 plan year, that classification applies to workers who earned more than $160,000 from the employer in the prior year.³ Plans run nondiscrimination tests comparing deferral rates of highly compensated employees against everyone else, and if the gap is too wide, excess contributions may be refunded. Safe harbor plan designs avoid this testing by requiring specific employer contributions.

Vesting Is Not the Same as Eligibility

Getting into the plan and owning the employer’s contributions are two different things. Money you contribute from your own paycheck is 100% yours immediately. Employer matching and other employer contributions often follow a vesting schedule that controls how much you keep if you leave early.

Federal law allows two standard vesting schedules for employer matching contributions in most plans. Three-year cliff vesting means you own 0% of the match until you complete three years of service, at which point you jump to 100% at once. Six-year graded vesting means you vest gradually, starting at 20% after two years and reaching 100% after six. Plans can always vest faster, and many do.⁸

Safe harbor 401(k) plans run on different rules. In a standard safe harbor design, all matching contributions must be 100% vested immediately. In a Qualified Automatic Contribution Arrangement, the match must vest fully after no more than two years.⁸ If you are weighing job offers, this is worth checking.

Rehired Employees and Breaks in Service

If you leave a job and come back, your prior service may still count. The general rule is that if you had any vested balance in the plan when you left, your earlier service must be credited on return. Because your own contributions are always vested, even a modest 401(k) balance preserves your service record. If you were already eligible to participate, the plan has to let you back in on your rehire date.

There is a narrow exception called the rule of parity. If you were 0% vested in employer contributions when you left and your consecutive break years (years with fewer than 500 hours) equal or exceed the years you had worked, the plan can reset your clock and treat you as new. In practice this requires at least five consecutive break years because of how current vesting schedules interact with the rule.

If you were not yet eligible but had built up some service credit, that prior service generally still counts toward eligibility when you come back, subject to the same rule-of-parity exception. Most rehires with meaningful tenure will qualify faster the second time around.

Eligibility Rules for Other Plan Types

Not every workplace plan is a 401(k). If your employer offers a SEP IRA, SIMPLE IRA, or 403(b), the eligibility rules change.

SEP IRA

Simplified Employee Pension plans are employer-funded IRAs common at small businesses. An employer must include any worker who meets all three conditions: age 21, service in at least three of the last five years, and a minimum amount of compensation.⁹ For 2026, the compensation threshold is $800.³ The employer can set easier terms but not stricter ones. Only the employer contributes to a SEP; employees do not defer their own salary. When the owner contributes a given percentage of pay for themselves, they have to contribute the same percentage for every eligible employee.¹⁰

SIMPLE IRA

The Savings Incentive Match Plan for Employees is available to employers with 100 or fewer employees. Eligibility turns on earnings rather than hours. You qualify if you earned at least $5,000 during any two prior calendar years (they do not have to be consecutive) and reasonably expect to earn at least $5,000 in the current year.¹¹ Employees can make their own salary reduction contributions, and the employer must also contribute, either as a dollar-for-dollar match up to 3% of compensation or a flat 2% nonelective contribution for every eligible employee. Employers cannot add waiting periods or earnings thresholds beyond what the statute allows.¹²

403(b)

Nonprofits, public schools, and certain religious organizations offer 403(b) plans. The structure is fundamentally different because of the universal availability requirement: if any employee can make salary deferrals, every employee must have that same opportunity.¹³ There is no one-year waiting period for employee deferrals.

A handful of narrow exceptions exist. Employers may exclude part-time employees who normally work fewer than 20 hours per week (defined as those not expected to reach 1,000 hours in any 12-month period), students performing services for the school where they are enrolled, employees already eligible for a 401(k) or 457(b) plan from the same employer, and nonresident aliens with no U.S.-source income. Generic labels like “temporary,” “seasonal,” or “adjunct” cannot be used to exclude someone; if an adjunct professor works enough hours to fall outside the 20-hour exception, the plan has to let them in.¹³ Employers also have to send a meaningful notice at least once a year telling eligible employees they can contribute.

If Your Employer Does Not Offer a Plan

You may still have access to a retirement account through your state. As of early 2026, roughly 21 states have enacted retirement savings programs aimed at private-sector workers whose employers do not provide a plan. Most run as automatic-enrollment IRA programs: the state provides the infrastructure, the employer processes the payroll deduction, and workers are enrolled by default with the right to opt out.

The employee count that triggers the employer’s obligation varies by state, running from as few as one employee to as many as 25. Penalties for noncompliance also vary and can reach $250 to $750 or more per employee per year in some states. If you have been told there is no plan at work, it is worth checking whether your state has launched one of these programs, because your employer may be required to participate.

1
Office of the Law Revision Counsel. 29 USC 1052 – Minimum Participation Standards
2
Internal Revenue Service. 401k Resource Guide Plan Participants Summary Plan Description
3
Internal Revenue Service. COLA Increases for Dollar Limitations on Benefits and Contributions
4
Internal Revenue Service. Retirement Topics – Automatic Enrollment
5
Office of the Law Revision Counsel. 26 USC 414A – Requirements Related to Automatic Enrollment
6
U.S. Department of Labor. Employee Retirement Income Security Act (ERISA)
7
Internal Revenue Service. Independent Contractor Self-Employed or Employee
8
Internal Revenue Service. Issue Snapshot – Vesting Schedules for Matching Contributions
9
Office of the Law Revision Counsel. 26 USC 408 – Individual Retirement Accounts
10
Internal Revenue Service. Simplified Employee Pension Plan (SEP)
11
U.S. Department of Labor. SIMPLE IRA Plans for Small Businesses
12
Internal Revenue Service. Retirement Plans FAQs Regarding SIMPLE IRA Plans
13
Internal Revenue Service. Issue Snapshot – 403b Plan – The Universal Availability Requirement