I always put my babies in the craziest terrain possible. It builds strength, bone density and a ballanced and sure footed youngster.
A youngster turned out in a rough paddock, develops so much more balance, stamina, muscle etc. than one turned out on a perfectly flat, square, manicured paddock.
My young warmblood has been turned out on a hill, with rocks and trees, since he was 12 months old. Yeah, he's had a few tumbles, but heck is he sure footed now, and very brave!
I like my young horse to have his time as a young horse, mucking around like a bat out of hell in the paddock. The owner of the other young horse in the paddock, tries to keep him rugged to the eyeballs, plaits his mane and tail etc so he doesn't rub it out, and flips out when my youngster chews her horse's mane, jumps on him etc. I couldn't care less, half of Billy's mane has been chewed out, he's full of bite marks, scrapes etc. But he LOVES being out there playing!!!