RE: Which gun should I give to a 15 year old?
Sorry about your loss.
The Mini-30 and the Rem. .308 can both be used for deer. I' d recommend the .308. A bolt-action is more appropriate for a beginner, (safer!!) and the .308 is a better round for deer and larger game than the 7.62X39mm. But the 7.62X39mm will work, if the right bullets are used.