My horse will be six in March, and my trainer sternly suggests I don't jump her more than once a week, lightly. "Less is more. Ground poles are fun too." I'm recommended not to jump over 18 inches and to just have fun over cross rails and little grids. My trainer has a five-year-old she retired from regular jumping after seeing his five-year-old half brother ruined by being jump too high and too soon.
After seeing that, she is probably a little more strict about jumping babies than most. But I see no disadvantage to her policies. It keeps a horse useful for longer and saves their joints. Jumping is tough. My friend's old jumping horse is a sorry creature. He's 20 years old. Even with joint injections, he isn't much use. He started showing signs of wear years ago. He was burnt out by too much jumping. Bad knees, bad hocks... Constantly lame.