> Some people just aren't cut out for athletics genetically. Some grow later on and may or may not be interested. Age seems like a fair sorting criteria and general enough to not introduce too much complexity.
Age isn't particularly fair but it isn't the worst either. As a kid, being around kids younger than you can be really displeasing. When you're a kid, the only thing you want is to be treated like you're older/better than you are. If you keep getting put on teams for people who are small - it's likely you're being put on teams for people who are smaller than you.
I had to play baseball with 7-yr olds for about 3 years. It wore me down as a kid. I gave up on baseball at that point and said I wasn't doing it anymore. The reason I had to play with them is because no coach wanted me on their team because they saw me as too small. (Small town, they didn't group by age as much as they did by size) I'd see kids go up to bigger-kid teams and get bigger but I stayed the same size.
As a kid - it was super disheartening to be stuck with the same 7 to 8 year olds for all that time and seeing people you used to play with in the bigger kid league. It sucked.
That said - baseball is probably the ONLY sport I can imagine being where you could move kids up by age and not by size as much. Football, soccer, basketball, etc. all involve a serious size component to them whereas baseball is less so - in my mind.
For reference - I grew up to be a normal sized adult. I was just super late on the curve compared to my peers.