I think it depends a lot on the maturity of the individual but at 15, your cousin is right about there. I've got a 16 year old on the
football team who is a solid 210lbs, he just squatted 225 for 20 yesterday. I've seen him rep 275 as well. I can push this kid with the weight on his back, though there are kids older than him that I'd rather see sticking to bodyweight squats.
I think what you've got planned is perfect. For now, technique work with the barbell, hard work with the bodyweight stuff.