Aja is one of the sons of Uttama (or Auttama) Manu, the lord of the third epoch (manvantara).

Brahmāṇḍapurāṇa (Maharṣi)(1.36.39)
Vāyupurāṇa(61.185; 62.9.34)
Viṣṇupurāṇa(3.1.15)
Mārkaṇḍeyapurāṇa(73.10)