33% of body weight is usually the practical limit for most people in moderately good shape if you want them to maintain a decent pace.

People in very good shape and crossing flat and easy ground can go to 50% and still maintain a decent pace for a time. For shorter periods of time a bit more but expect a certain number to come up lame. Particularly if the footing is uneven at those weights the number of turned ankles and blown out knees will multiply. Loaded too heavy for their abilities the amount of time spent on rest breaks, the effort needed to treat and transport injuries, and the slow pace will erase any benefit of the higher loading. Two or three trips with a lighter loads can often move more cargo than taking it all in one go.

Pathfinders and scouts maintain the highest mobility by keeping the weight down to between 5% and 10% of body weight and sleeping cold and limiting food and water. This is also a good estimate for children, and the old.

As an aside, a friend claimed that %25 was about as high as you want to go for a dog. His large, roughly 60 pound mutt, had a pack that ran about 15 pounds with food, water and some doggy gear like booties, bowl and a sleeping pad. The dog seemed to love to backpack and would beg for someone to put his pack on.