You can start a 401(k) as soon as you meet your employer’s eligibility rules, and under federal law those rules can’t make you wait past age 21 or more than one year of service. Many plans let you in sooner. The practical answer to when to start a 401(k) is almost always the first day you’re eligible, because employer matching and tax-deferred growth only work on money that’s already in the account.
The Earliest You Can Legally Join
ERISA caps how long an employer can keep you out. A plan cannot require you to be older than 21, and it cannot require more than one year of service before you become eligible.1U.S. Department of Labor. FAQs about Retirement Plans and ERISA A year of service means a 12-month period in which you complete at least 1,000 hours of work, roughly 20 hours per week.
These are ceilings, not floors. Plenty of employers let you enroll on your first day, and some allow employees under 21 to participate.1U.S. Department of Labor. FAQs about Retirement Plans and ERISA One narrow exception exists at the other end: a plan can require two full years of service, but only if it gives you immediate 100% vesting in employer contributions the moment you enter.2Internal Revenue Service. Retirement Topics – Vesting Most employers stick with the one-year rule so they can spread vesting of matching contributions over as long as six years.3Internal Revenue Service. Issue Snapshot – Vesting Schedules for Matching Contributions
Check your plan’s summary plan description or ask HR. The document will tell you the exact age, service, and hours required at your employer.
Part-Time Workers and the 500-Hour Rule
If you work part-time and never hit 1,000 hours in a year, you’re no longer locked out. Under the long-term, part-time employee rule created by the SECURE Act and expanded by SECURE 2.0, employers must let you make salary deferrals once you complete two consecutive 12-month periods with at least 500 hours in each, starting with plan years beginning in 2025.4Federal Register. Long-Term, Part-Time Employee Rules for Cash or Deferred Arrangements Under Section 401(k) You still have to meet the plan’s minimum age requirement (up to 21) by the end of that second 12-month period.
Only periods starting on or after January 1, 2021, count toward the calculation.4Federal Register. Long-Term, Part-Time Employee Rules for Cash or Deferred Arrangements Under Section 401(k) If you work roughly 10 hours a week year-round, you’re likely on track.
When Contributions Actually Begin
Meeting the requirements doesn’t mean deferrals start the next paycheck. Federal law requires your employer to admit you no later than the earlier of two dates: the first day of the next plan year, or six months after you satisfy eligibility.5Internal Revenue Service. A Guide to Common Qualified Plan Requirements Within that window, the plan document sets specific entry dates, commonly the first of each month or calendar quarter.
If your one-year anniversary lands on March 15 and the plan uses quarterly entry, you’re in on April 1. You typically have to complete enrollment paperwork or make an online election before the entry date. Miss it and you slide to the next one, losing weeks or months of contributions.
Automatic Enrollment in Newer Plans
Under SECURE 2.0, 401(k) plans established on or after December 29, 2022, must automatically enroll eligible employees. The default deferral has to be between 3% and 10% of your pay and must increase by 1% each year until it reaches at least 10%, capped at 15%.6Internal Revenue Service. 401(k) Plan – Overview You can change the rate or opt out.
Several employers are exempt:
- Plans established before December 29, 2022
- Businesses in existence fewer than three years
- Employers with 10 or fewer employees
- Church and government plans
If your plan predates the rule, enrollment is on you. Waiting for someone to sign you up is how people lose their first year of matching.
How Much You Can Put In Once You Start
For 2026, the employee deferral limit is $24,500.7Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 That’s your salary deferrals only; employer matching and profit-sharing sit on top. The maximum compensation the plan can consider for you is $360,000.8Internal Revenue Service. COLA Increases for Dollar Limitations on Benefits and Contributions
If you’re 50 or older, catch-up contributions raise the ceiling:
- Ages 50 to 59, or 64 and older: up to $8,000 extra, for a total of $32,500.7Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
- Ages 60 to 63: up to $11,250 extra, for a total of $35,750, under the SECURE 2.0 “super catch-up.”7Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
Starting in 2026, if your prior-year wages from the sponsoring employer exceeded $150,000, your catch-up contributions generally must be designated as Roth (after-tax). The dollar amount you can contribute doesn’t change; only the tax treatment does.
One deadline detail matters at the calendar edge: unlike an IRA, 401(k) deferrals must come out of paychecks issued by December 31 to count for that year. You can’t make a January lump sum and apply it back. If you’re aiming for the full limit, spread it across your remaining pay periods and submit changes before the last payroll cycle.
You Can Only Contribute From a Paycheck
A 401(k) contribution has to come out of wages your employer pays you, through payroll. You cannot fund one from personal savings, investment income, or earnings at another job.9Internal Revenue Service. 401(k) Plan Fix-It Guide – You Didn’t Use the Plan Definition of Compensation Correctly for All Deferrals and Allocations Eligible compensation generally includes salary, bonuses, commissions, tips, and certain fringe benefits.
Take an unpaid leave and your contributions pause because there’s no paycheck to defer from. They resume when you’re back on payroll.
If Student Loans Are the Reason You’re Delaying
If you’re paying down student loans and feel you can’t spare payroll deferrals, your employer may still be able to fund your account. Under SECURE 2.0, employers can make matching contributions based on your qualified student loan payments, even when you defer no salary at all.10Internal Revenue Service. Guidance Under Section 110 of the SECURE 2.0 Act with Respect to Matching Contributions Made on Account of Qualified Student Loan Payments The rate has to match the plan’s regular deferral match, and it must be offered to everyone eligible for that match.
You certify annually that you made qualifying loan payments; the plan can rely on your certification without further documentation.10Internal Revenue Service. Guidance Under Section 110 of the SECURE 2.0 Act with Respect to Matching Contributions Made on Account of Qualified Student Loan Payments Not every employer has added the feature, so ask.
The Saver’s Credit if Your Income Is Modest
Starting a 401(k) at lower income levels can pull down an extra tax break, the Retirement Savings Contributions Credit. The 2026 income limits:
- Married filing jointly: up to $80,500
- Head of household: up to $60,375
- Single or married filing separately: up to $40,2507Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
The credit is worth 10%, 20%, or 50% of the first $2,000 you contribute ($4,000 if married filing jointly), depending on income. It cuts your tax bill directly rather than reducing taxable income. Beginning with the 2027 tax year, SECURE 2.0 replaces the credit with a “Saver’s Match” deposited into your retirement account.11Internal Revenue Service. Credit for Qualified Retirement Savings Contributions For 2026, the credit still works the traditional way on your return.
Is There an Age When It’s Too Late?
No. There is no upper age limit on 401(k) contributions. If you’re working and getting paid, you can keep deferring and keep receiving any employer match, whether you’re 65 or 85.
The wrinkle for older workers is Required Minimum Distributions. The starting age depends on your birth year:
- Born 1951 through 1959: RMDs begin at age 73.
- Born 1960 or later: RMDs begin at age 75.
A “still-working” exception applies to your current employer’s plan. If you don’t own more than 5% of the business sponsoring it, you can delay RMDs from that plan until the year you actually retire.12Internal Revenue Service. Retirement Plan and IRA Required Minimum Distributions FAQs The exception covers only the current employer’s 401(k), not IRAs and not old 401(k)s from prior jobs. For someone still working past 73 or 75, that’s a real reason to keep contributing rather than roll the account over.