Joe De Sena recommends starting kids in gymnastics young for body awareness and coordination, then exposing them to seven or eight different sports around ages 7-8 before specializing by 11-12. He also suggests enrolling all siblings in the same sport to cut down on logistics.
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For anybody listening who has kids or is about to have kids, if they're young, my belief now after raising 4 kids is gymnastics creates incredible body awareness, incredible strength, no risk of injury. You're not lifting weights, you're just using body weight, becoming really coordinated. And then when they're like 7, 8 years old, I would put them in every possible sport. They would play 7, 8 sports if they can, everything. And I would do that for like 3 years. Now they're 11 and 12. They're coordinated. You got a chance to see what sport they excel at, and then I would go all in. And in a perfect world, if you have 4 kids like me, you put them all in the same sport. Not because you want to take anything away from any one of them, but because the family can't handle effectively going in 4 different directions every day. Just can't— you just can't do it. Everybody suffers a little bit, especially the parents.