My 8yo DD is starting her orthodontic treatment tomorrow. Her mouth looks like a jack-o-lantern. She will be starting with a plate and them more than likely braces after that. In a few months DS1 (who will be 10) will also start, but our Dentist wanted to wait till he had turned 10 to see if he had had a growth spurt to give the jaw a chance to grow a little more. 8 months ago when we first discussed orthodontics he said he wanted to wait till she was 8. I have no idea though if there is an absolute minimum age to start though sorry.