Mahavagga, Khandaka 1, Chapter 75

At that time the venerable Kumārakassapa had received the upasampadā ordination when he had completed the twentieth year from his conception (but not from his birth). Now the venerable Kumārakassapa thought: 'The Blessed One has forbidden us to confer the upasampadā ordination on persons under twenty years of age^[1], and I have completed my twentieth year (only) from my conception. Have I, therefore, received the upasampadā ordination, or have I not received it?'

They told this thing to the Blessed One.

“When, O Bhikkhus, in the womb the first thought rises up (in the nascent being), the first consciousness manifests itself, according to this the (true) birth should be reckoned. I allow you, O Bhikkhus, to confer the upasampadā ordination on persons that have completed the twentieth year from their conception (only).”