And Michael Phelps has been training with his coach for 16 years.
Not sure which club Gabby switched from when she was 14, to go to Chow's Gymnastics club, but gymnastics coaching quality is very important not just for advancement of skills, but safety. As is the gymnastics equipment - pits for safe landing of hard skills as you learn them, a good spot, coaches teaching correct technique and steps/progressions to learn it, and lots of mats, that are not hollowed out.