Hi, Rachel. My husband's a university prof in exercise physiology and we really debated this one when our daughter showed early signs of aptitude for ballet. What we decided is that, with a skill-specific activity like dance, if you can find a program that will teach it in a relatively low-key, low-stress way, a child can easily and happily absorb a ton of information in the mind AND the muscles starting at about age 5 or 6. And we believe that keeping other activities (like sports) in the mix during the elementary-school ages is essential for developing stamina and avoiding burnout. Then, between the ages of 12 and 14, you can begin increasing the intensity to something more "career track," if the interest and aptitude are still there. This philosophy is replicated in swimming and diving training, which is also very skill-specific, especially in the younger ages. The sooner you can make skills a part of muscle memory, the easier it is to build on them later. But that is NOT to ever say that a dancer who starts training later couldn't end up with the same skills by the age of 17 or 18. And I know the Bolshoi program doesn't let kids start any dancing until age 10.
The biggest problem we've found in sticking to this, though, is finding teachers who will keep things low-key and low-stress before the age of 12. So many of them get that heady whiff of talent and it's "goodbye, childhood, hello, full-time training!" In our experience -- and our daughter is now 11 -- it's only been about one-third of her teachers who really get and respect the "slow boil" philosophy of training. I was just telling a dance-mom friend yesterday that I'm beginning to suspect (hope?) that the 12-and-under ages are the hardest ones for a dance parent to navigate. I feel like I"m just constantly saying "no" to people's demands for more from my daughter.