Does anyone know what code section the in-service withdrawal for participants aged 59 1/2 rules can be found?

Code Section 401(k) says that that those elective deferrals may only be distributed upon certain events, one of which is attainment of age 59-1/2. Code Section 403(B) has a similar restriction. The 10% excise tax for distributions prior to age 59-1/2 (with various exceptions) is in Code Section 72(t). There's an old regulation under 401 that says profit-sharing money may be distributed upon the attainment of a specified age.

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