I knew people always prefered them to be a certain age, but I always thought the riding/training, and the temperment were more important.
My dad's older TB, he was 20 when we got him, I would have been perfectly fine letting a kid ride him at a walk in a roundpen, and with training he would have made an amazing kids horse. He was the sweetest thing, never spooked, never bucked kicked bit, nothing. I loved him to death.
My mare, was being advertised as a kid-broke horse. Wanna know how old she was? 1yr 8months. Kidbroke? She shouldnt have even been broke! My parents bought her for me as a surprise. I cant even remember how many times that little brat has bucked me off for no reason at all! (she was of course given time to mature though)
My gelding? I bought unbroke, was told he was loco, untrainable, I should ship him off to slaughter ect. He is amazing. He doesnt act up, he'll test me by refusing to move. Thats it. He is a puppydog, and when Im injured or sick, he takes such good care of me. With more time (he's only 5, with about a year of training undersaddle) I would totally trust him to take care of a kid.
Never forgot the horse that taught you to be fearless (Braveheart RIP) the horse that taught you to stay strong no matter how bumpy the road is (Caesar) and the horse that always had to test gravity by throwing you in the air (Cali)