If Hiranyakashipu performed austerity for 10,000 years, how was Prahlad just 5 years old when he returned?

From: Deepak K

I heard that when Hiranyakasipu was meditating, A bird was chanting the name of Narayana and he got disturbed, So he came back to Kayadhu and concieved Prahlad in her womb. Then Hiranyakasipu again went for 10000 years meditation and returned back, Prahlad was just 5 years old… How is this possible?