Shatapatha Brahmana 11.4.1.7

11.4.1.7

sa vai gautamasya putra vṛto janaṃ dhāvayet yastaddarśapūrṇamāsayorvidyādyasmātkumārasya retaḥ siktaṃ na sambhavati yasmādasya madhyame vayasi sambhavati yasmādasya punaruttame vayasi na sambhavati

'He alone, O son of Gautama, may drive about amongst people as chosen (priest), who knows from the Full and New-moon sacrifices whereby the seed of the boy is not productive, whereby in his middle age it is productive, and whereby again in his last stage of life it is not productive;--