No one is allowed to keep retirement funds in their account forever. For 401(k) plans, you generally must begin distributions no later than April 1 of the calendar year in which you either reach age 72 or retire. For that first year, you will have two required distribution dates. One withdrawal will take place on April 1st and an additional withdrawal must be made by December 31st. To calculate your required minimum distributions for the following years, the IRS has provided a chart here.