At least age 21 and
Have at least 1 year of service
a plan other than a 401(k) plan can require an employee to accrue 2 years of service, but if it does, then the employee is immediately vested in their accrued benefit)
The employer must follow the eligibility rules in the plan document, which can be less restrictive than those listed above.
Example: Employer B’s plan allows employees to participate in its profit-sharing plan at age 18.
Employees should review the Summary Plan Description that their employer gives to them to determine the eligibility requirements for their plan.
Publication 560, Retirement Plans for Small Business (SEP, SIMPLE, and Qualified Plans)
Publication 4222, 401(k) Plans for Small Businesses
Publication 4531, 401(k) Plan Checklist
Video: Retirement Plans and the Misclassification of Workers.