i think it stunts your growth a little. i started gym when i was 3. by the time i was 8 or 9 until i was 13 or 14, i was doing 5 to 6 days, 20 to eventually 30 hours a week of heavy competitive gymnastics. i was 4' 9" when i quit and had started to develop small hairline fractures in some of my growth plates. after i quit, my fractures healed very quickly and i grew more and finally started puberty (i was 15). i also grew a lot, i'm just over 5' now. my doctor said i probably would have grown to my current height much younger if i weren't doing gymnastics, and maybe a bit taller than if i hadn't injured my growth plates. this is compared to my little sister who was 5' 3" when she was 14 (the age when i was 4' 9") and now at almost 16 is almost 5' 5". my mom is a little shorter than my sister just to put it in perspective. not like i'm from a family of giants, but it seems gymnastics did slightly affect my growth. so yeah, that's my experience. doing competitive gymnastics my whole life, especially while i was growing, seemed to effect that. but that's b/c i was at the age when my growth plates hadn't set. at 21, you probably just have a little bit i more to grow, and if you're just starting, you won't be doing the very physically demanding and time intensive stuff that would also affect growth.