Shatapatha Brahmana 11.4.1.14

11.4.1.14

atha yadbarhi stṛṇāti tasmādimāḥ prajā lomaśā jāyante'tha yatpunariva prastaraṃ stṛṇāti tasmādāsām punariva śmaśrūṇyaupapakṣyāṇi durbīriṇāni jāyante'tha yatkevalamevāgre prastaramanupraharati tasmācīrṣaṇyevāgre palito bhavatyatha yatsarvameva barhiranupraharati tasmātpunaruttame vayasi sarva eva palito bhavati

And inasmuch as he spreads a cover of sacrificial grass (on the Vedi), therefore creatures here are born with hair; and inasmuch as he for the second time, as it were, spreads the Prastara-bunch, therefore, for the second time, as it were, the hair of the beard and the arm-pits, and other parts of the body grow; and inasmuch as at first he only throws the Prastara-bunch after (the oblations into the fire), therefore it is on the head that one first becomes grey; and inasmuch as he then throws after it all the sacrificial grass of the altar-ground, therefore, in the last stage of life, one again becomes grey all over.