I'll talk about llamas because they mature at the same rate as horses. They're subject to the same foot, leg, and tendon damage as horses if worked too young.
Last year while packing I took along a young stud about 9 mo old. He just tagged along and was a general nuisance. This year as a yearling, he will carry a saddle only (about 5 lb). He wouldn't even get that but he needs experience in the middle of a pack string and I need to tie others behind him. As a 2 yo, he'll carry maybe 20 to 30 lb.
I have a 3 yo that'll get 40 to 50 lb this year. He's already the biggest llama I've got but that doesn't make his legs mature. He still needs to go light.
As a 4 yo, they can get a full load of up to 100 lb if they're in good enough shape to handle it. For most that are used only casually, 80 is better.
