It depends on what you want to do while backpacking. If you are on flat trails, you can carry more. I do not find weight affects me much when I am just walking on flat ground. But if you are doing 3000 feet gain in a day, particularly at altitude, you have to lighten up. If you are going to cross-country travel over rough terrain or climb class 2-3 passes, you have to go even lighter or the load will throw you off your balance. Do you want to walk lots of miles each day? Then packs need to be light. Do you want short travel days and more time in camp and more camp comfort? Then you can carry more.
Whatever, it is much more fun to go as light as you can. Just because you "can" carry 50 pounds does not mean you "should" carry 50 pounds. With today's lighter gear, your base weight (weight of pack without food, fuel or water) can easily be around 20-25 pounds without breaking the bank on high dollar UL gear. Food should be no less than 1 pound per person per day and no more than 2 pounds per person per day. When I took my 17-year old daughter on a long backpack I carried more weight than her, even though she was bigger (taller) and weighted more. You do not want to get your kid hating backpacking. If your teen is reluctant, the worst thing you can do is load him down.
Not to exceed 25%-30% "lean" body weight is a good rule of thumb for me. I weight 115 and get very uncomfortable with packs more than 40 pounds. Not that I cannot do it, just makes the trip less fun. I aim for 18-20 pounds without food and water and 1.3 pounds (2,500 calories per day) food per day.