about gymnastic twins where one quit young enough and grew 6 inches taller than the other.

Sounds legit. My daughter reckons the only reason she got taller than me was because of the stupid panpanic otherwise she probably would have stayed shorter. Similarly eldest is going to get surpassed in height by youngest (who quit gymnastics when he was between 8 and 10) probably soon.

Have you ever seen someone six foot on the balance beam?

Yep, the boys when we used to do switch comp XD None of our girls are that tall.