I've been using 15% for my grandkids (oldest getting close to 12). I wouldn't use more than 20% for a teenager who is still growing. A lot depends on physical conditioning, amount of body fat, maturity of joints and other factors.

20% of my ideal weight is about the maximum amount I can carry. Above that my knees and feet start screaming!
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey