re: Good mile time for a kid

I would not try to overthink what your 9 year old is doing. As he grows he should naturally get more athletic as long as he's participating in sports/being active. He has energy now and should still have it leading into his college years. He will get bigger (weight and height) so he will have to carry more with him as he runs. His form may change. It may benefit him. All these changes will impact him down the road.

If anything, just encourage him to do things that he likes. I didn't really like track that much when I was growing up. I really just wanted to beat people and happened to be fairly decent at it further into my HS years. Ended up walking on in college and enjoyed competition.