It's not fair or right at some point to treat kids the same. Some are better. At school, we put the smart kids in accelerated classes. Plus at some point, you put a kid who is a bad athlete at pitcher or catcher and he looks terrible. And it's your fault because you set him up to fail.
I remember having a parent on one of my little league football teams complain about his kid not getting the chance to play running back. The kid was terrible. I was afraid he was going to get hurt. I finally relented and gave the kid a chance to run the ball. He got drilled by a couple of big defensive linemen. He had to be helped off the field, crying. Hope his dad learned a valuable lesson.
I'd say 8 or 9 is when you can start treating the kids differently.