26 CFR §1.410(a)-4

1. (a)Maximum age conditions
   1. (1)General rule. A plan is not a qualified plan (and a trust forming a part of such plan is not a qualified trust) if the plan excludes from participation (on the basis of age) an employee who has attained an age specified by the plan unless—
      1. (i)The plan is a defined benefit plan or a target benefit plan, and
      2. (ii)The employee begins employment with the employer after the employee has attained an age specified by the plan, which age is not more than 5 years before normal retirement age (within the meaning of section 411(a)(8) and § 1.411(a)-7.
   2. (2)Examples. The rules provided by this paragraph are illustrated by the following examples:
2. (b)Time of participation
   1. (1)General rule. A plan is not a qualified plan (and a trust forming a part of such plan is not a qualified trust) unless under the plan any employee who has satisfied the applicable minimum age and service requirements specified in § 1.410(a)-3, and who is otherwise entitled to participate in the plan, commences participation in the plan no later than the earlier of—
      1. (i)The first day of the first plan year beginning after the date on which such employee first satisfied such requirements, or
      2. (ii)The date 6 months after the date on which he first satisfied such requirements,
   2. (2)Examples. The rules provided by this paragraph are illustrated by the following examples: