The Compendium of Caraka 1.5.82

1.5.82

balaṃ śiraḥkapālānāṃ viśeṣeṇābhivardhate/
dṛḍhamūlāśca dīrghāśca kṛṣṇāḥ keśā bhavanti ca//

The strength of the cranial bones especially increases, / and hair becomes firmly rooted, long, and black;