RE: First Rifle for 12 year old...
Another vote for the .243. My first buck was a 10 pointer with beautiful tall tines - quite a benchmark to set for myself!
I'd also suggest good hearing protection, and lots of practice. We never bothered with the "ears" on the farm, and it took me years to realize that the .243 didn't have enough recoil to make me flinch, but it sure made enough noise to do so!
Good luck!