The arm naturally pronates after release so throwing a pitch that involves pronation means the arm is rotating the same direction it will rotate after release - it's just starting that rotation sooner. There's no harm in that so long as the pitcher pronates only to his comfort level.
The circle change is probably the most common pitch thrown using pronation. Other pitches that use pronation are the sinker and the screwball.
In my opinion, no age is too early to learn a change-up. With a circle change, the cue I often use is to tell the pitcher to get his middle and ring fingers on top of the ball just as the index and middle fingers would for a fastball. However, young kids have small hands and, therefore, are often not able to make use of the circle grip. So they need to use a 3-finger or palm grip instead. But they can still throw with a bit of pronation (again, just to their comfort level).