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Is there legal reason for restricting someone under 59-1/2 from an in-service rollover from a 401K to an IRA?
You're going to find a lot of conflicting or vague answers on the internet because there are a lot of plan design elements that are set by the plan sponsor (employer).  There are laws that mandate certain elements and dictate certain requirements of plan sponsors, many of these laws are related to record keeping and fiduciary duty.  There is a lot of latitude for plan sponsors to allow or restrict employee actions even if there is no law against that activity.   There are different rules mandated for employee pre-tax contributions, employee post-tax contributions, and employer contributions.  You have more flexibility with regard to the employer contributions and any post tax contributions you may have made; your plan may allow an in-service distribution of those two items before you reach age 59.5. While your HR department (like most -all- HR departments) is not staffed with ERISA attorneys and CPAs it is your HR department and applicable plan documents that will lay out what an employee is permitted to do under the plan.