A 401(k) plan cannot make you wait past age 21 or more than one year of service to start contributing, and a “year of service” means a 12-month stretch in which you worked at least 1,000 hours.1Internal Revenue Service. 401(k) Plan Qualification Requirements Those are the federal ceilings on 401(k) eligibility requirements. Your employer can set the bar lower — day-one eligibility, three months, six months — but never higher. The wrinkles come from part-time schedules, rehires, automatic enrollment mandates for newer plans, and a narrow exception that lets some plans stretch the service requirement to two years.
The Federal Ceilings: Age 21 and One Year of Service
To be eligible under the strictest allowable plan, you need to hit both marks: age 21 and one year of service with at least 1,000 hours worked in a 12-month period.2Office of the Law Revision Counsel. 26 USC 410 – Minimum Participation Standards Turning 21 alone isn’t enough if you haven’t finished the service year, and finishing the service year isn’t enough if you’re still 20. A plan that required employees to be 25, or to wait 18 months, would violate the tax code and risk losing its qualified status.3Internal Revenue Service. Retirement Topics – Significant Ages for Retirement Plan Participants
Employers have wide latitude to be more generous. Some open the plan to every new hire on day one. Others use a three- or six-month service period, or allow immediate elective deferrals but hold the employer match back for a while. Whatever the plan chooses, the terms have to be in the plan document and applied the same way to everyone in the same category.
The Two-Year Service Exception
One narrow situation lets a plan push the service requirement to two years: employer contributions that vest 100% immediately. If matching and profit-sharing dollars are fully yours the moment they land in your account, the plan can make you wait up to two years of service before those contributions begin.1Internal Revenue Service. 401(k) Plan Qualification Requirements Even then, the plan still has to let you make your own elective deferrals after no more than one year of service. Most plans don’t use this option because running two eligibility tracks adds administrative work.
When You Actually Start Contributing
Clearing the age and service requirements doesn’t put money in the plan the next Friday. Plans have “entry dates” — the scheduled dates when newly eligible employees actually start participating. Federal law caps the wait: your participation must begin no later than the earlier of the first day of the plan year after you met the requirements, or six months after you met them.4Office of the Law Revision Counsel. 26 USC 410 – Minimum Participation Standards – Section 410(a)(4)
Most plans use semi-annual entry dates of January 1 and July 1 to stay inside that window. If you satisfy the eligibility requirements on March 10, the plan has to let you in by the earlier of the next January 1 or September 10, and a July 1 entry date would typically pick you up. Some plans use quarterly or monthly entry dates, and a plan can offer entry the moment eligibility is met. Six months is the outer limit.
Part-Time Employees
Part-time workers who never hit 1,000 hours in a year used to be shut out permanently. The SECURE Act and SECURE 2.0 changed that. Starting in 2025, a plan must let you make elective deferrals if you’ve worked at least 500 hours in each of two consecutive 12-month periods and reached age 21.5Office of the Law Revision Counsel. 26 USC 401 – Qualified Pension, Profit-Sharing, and Stock Bonus Plans – Section 401(k)(15) The original SECURE Act set this at three consecutive years with counting beginning in 2021; SECURE 2.0 shortened it to two.
There’s a catch. Your employer is not required to make matching or nonelective contributions on your behalf as a long-term part-time employee, even if full-time participants get them.6Office of the Law Revision Counsel. 26 USC 401 – Qualified Pension, Profit-Sharing, and Stock Bonus Plans – Section 401(k)(15)(B) Some employers extend the match voluntarily, but the law only guarantees you the right to defer your own pay.
Automatic Enrollment at Newer Plans
If your employer set up its 401(k) on or after December 29, 2022, the plan must automatically enroll eligible employees for plan years beginning after December 31, 2024.7Federal Register. Automatic Enrollment Requirements Under Section 414A You’ll be enrolled by default unless you opt out. Plans that already existed before that date are grandfathered.
The default deferral rate must be at least 3% of pay but no more than 10%, and it steps up by one percentage point each year until it reaches at least 10%. You can change the rate or opt out entirely at any time. Several categories of employer are exempt from the mandate: businesses in existence fewer than three years, employers with fewer than 10 employees, church plans, and government plans.
Rehires and Breaks in Service
Leaving and coming back raises the question of whether the eligibility clock restarts. A “break in service” happens when you complete 500 or fewer hours in a 12-month computation period.8eCFR. 29 CFR 2530.200b-4 – One-Year Break in Service
If you were vested in the plan — meaning you had a nonforfeitable right to at least some employer money — the plan must credit your prior service when you’re rehired, no matter how long you were gone. If you were not vested but had fewer than five consecutive one-year breaks, the plan still has to count your earlier service. Only when a non-vested employee has five or more consecutive one-year breaks can prior service be disregarded. So a worker who left after 10 unvested months and returned three years later still gets credit for those 10 months; the same worker returning after six years could be treated as brand new.
Who Can Be Excluded Entirely
Some categories of workers can be left out of a plan without violating the coverage rules. The two most common are collectively bargained employees and certain nonresident aliens.
If you’re covered by a collective bargaining agreement and retirement benefits were on the table during negotiations, the employer’s general 401(k) plan can exclude you. Union workers typically have separate retirement arrangements negotiated through the contract. Nonresident aliens who receive no earned income from U.S. sources can also be excluded.9Office of the Law Revision Counsel. 26 USC 410 – Minimum Participation Standards – Section 410(b)(3)(C) That’s narrower than it sounds: a nonresident alien earning wages from a U.S. employer generally does have U.S.-source income and can’t be excluded on this basis.
What Happens If You Were Wrongly Left Out
Eligibility mistakes are common. An HR system skips a new hire, a start date is entered wrong, a rehire’s prior service isn’t credited. When an eligible employee is wrongly excluded, the employer has to fix it, and the fix costs real money.
The standard correction is a qualified nonelective contribution equal to 50% of the missed deferral opportunity, calculated by multiplying the average deferral percentage for the employee’s group by the employee’s compensation for the year they were excluded. On an $80,000 salary in a group averaging 8% deferrals, the missed deferral is $6,400 and the employer owes $3,200, plus earnings. If the employer catches the mistake promptly and meets certain conditions, including notifying the affected employee within 45 days, the corrective contribution drops to 25% of the missed deferral.10Internal Revenue Service. 401(k) Plan Fix-It Guide – Eligible Employees Weren’t Given the Opportunity to Make an Elective Deferral Election On top of that, the employer must fund any match or nonelective contribution the employee would have received. Corrective contributions vest immediately.
Employers can self-correct through the IRS’s Employee Plans Compliance Resolution System without filing anything or paying a fee, provided they document the fix and adjust procedures.11Internal Revenue Service. Steps to Self-Correct Retirement Plan Errors If you think you were wrongly excluded, raising it with HR or the plan administrator is the fastest way to a correction. Employers have strong reasons to fix these before an IRS audit does it for them.