I think in this matter it has nothing to do with age.
This is 100% maturetie and thrust.
You are responsible whom ever you handover guns and amo to.
There has to be a purpose to carry guns, to me thats only two reasons target practise or hunting.
Started off at age 10 my selfe with a boltaction 22lr, and advanced to 410 shotgun witthin a year.
I was never aloud with friends, allways by myself and had to acount for ewery single shott fired, clean the gun, gut and particepate in handeling the game all the way, to the dinner table.
Including eating whatever I brought home.
As for your 15 years old, you got to train him until you have a mutual thrust on gun handeling, that thrust may never come, if so newer let him handle your guns on his own.

PAUL