Apastamba dharma SUtra prescribes upanayana samskAra only for the first three VarNas.
For BrAhmaNa eight years from conception during vasanta Rtu
For Kshatriya eleven years from conception during GrIshmaa Rtu
For VaiSya twelve years from conception during Sarad Rtu
For BrAhmaNa eight years from conception during vasanta Rtu
For Kshatriya eleven years from conception during GrIshmaa Rtu
For VaiSya twelve years from conception during Sarad Rtu
vasante brāhmaṇam upanayīta grīṣme rājanyaṃ śaradi vaiśyaṃ
garbhāṣṭhameṣu brāhmaṇaṃ garbhaikādaśeṣu rājanyaṃ garbhadvādaśeṣu vaiśyam !
garbhāṣṭhameṣu brāhmaṇaṃ garbhaikādaśeṣu rājanyaṃ garbhadvādaśeṣu vaiśyam !
------ Apastamba dharma sUtra 19