Je ne suis pas né ici mais j'y ai passé toute ma prime enfance.

I think this is a mistake — prime is not childhood by itself?

“La prime enfance” is a common idiom which means the first years of your life, the early childhood.

“Prime” is an adjective whose meaning is “premier” but I actually had to look it up because in my experience it’s rare outside of idioms like this one.