Manava Dharmashastra 2.38

2.38

ā ṣodaśād brāhmaṇasya sāvitrī nātivartate
ā dvāviṃśāt kṣatrabandhor ā caturviṃśater viśaḥ //

The (time for the) Savitri (initiation) of a Brahmana does not pass until the completion of the sixteenth year (after conception), of a Kshatriya until the completion of the twenty-second, and of a Vaisya until the completion of the twenty-fourth.