I usually wait until the child is about 18 or so, depending upon if he is still growing or not. Best to be seen in person.
Gynecomastia will subside in many boys who develop it within 2 years of onset. If it does not go away within that amount of time, it rarely will. I always wait for that 2 year period before operating. I have done this on boys as young as 14. Inverted nipples can easily be repaired at the same time as gynecomastia removal. Best of luck.