Youth boxing?

My daughter wrestles most of the time and is also doing some Judo and BJJ. She became fascinated with Boxing and Muay Thai and I found a gym that has had a great deal of success helping girls do well. Trying to balance the time for everything involved. A lot of people say that the best boxers are almost always people who start very young.



I guess the crux of my question is, how critical is it that she start young if she wants to be successful later?