I think 25% is a good figure for the average young person up to the age of about 15 or 16. Over that I think up to 33% is the supposed figure that is quoted commonly is South Australia, and probably isnt too far from the truth for the average fit person.
With proper training you can certainly go higher. Ive been training lately with packs up to 59.1kg. I weigh in at 73.5-75kg, depending on what Ive eaten, drunken, when I last went to the toilet, etc. So that means 78.8 to 80.4% of my body mass. A fair bit more than both those figures above. And I am by no means an elite athlete. I am sure if you got a proffesional mountaineer or special forces soldier they would scoff at the % I am doing and run away from me up a hill with it on.
I think the important thing is to limit pack weights as much as possible, especially with young, developing bodies who haven't really seen any muscular and skeletal stresses of that sort before. I think the 60lb figure that keeps floating around for NOLs packs is definitely way too much. I remember at 16 trying going for week long bushwalks with 22kg (lots of water) thinking "Oh my god, my body is going to break". Then when we loaded up with 5L more water I almost broke down and cried. And while I was pretty skinny, I was by no means unfit for my age. A couple of years on and I had heard about trips to South West Tasmania which required packs of 35kg, due to the harsh conditions gear, 17 days food, lilos for river rafting, etc. When I tried carrying that I thought it impossible. Now I dont even notice 35kg. Its certainly all relative to the individual.