My 10 year is shooting my 6.5 Grendel and he is very small for his age. He killed a coyote facing him at 125 yards the other night so I feel like it is a very suitable caliber for a youth. With the adjustable AR15 stock the Grendel is a good choice for a youth and an adult as well. If Larue can kill an elk with a 6.5 Grendel at 400 yards then the caliber is more than adequate.
Also, it covers everything from a prairie dog, coyote, Hog, deer and as noted an elk. Another plus I see is that the 6.5 Creedmoor is very popular as well so the 6.5 bullets should continue to be readily available.