A couple years ago I took my 9 year old on a 10 mile day hike. It was pretty much flat the whole way. 8 miles seemed pretty easy, 9 seemed doable, but 10 was just too far for him at that time. So, my recommendation is no more than 10. Of course, once he is 13 or 14, then you can just plan as far as you want to go, and he will go that far for sure. At 16, you need to put some of your stuff in his pack....that way you will be able to keep up.