Good question. I'm not entirely sure. My basketball hoop can lower to 7.5 feet, and my little brothers enjoy playing on the lower hoop. Sometimes, I play with them, but I avoid shooting jumpers and try to dunk or make lay-ups the whole game.
Not really, for most young shooters, who havent developed the strength to shoot 10 ft without pushing the ball, should shoot on lower rims to develop their shot, it also helps some players develop a lighter touch, as long as you can adjust to the 10ft rim later, it shouldnt hurt you that much, and can even help you.