So many variables. Fenced in yard vs. non. Busy streets. Older siblings. Etc?
Probably elementary aged for my oldest but we lived in town with no fence.
For my youngest probably as soon as he could be outside.
But we live in the country. Huge yard. Quiet dead end street. And lots of older siblings to watch over him.
As for the "too old" side. It gets less and less as they get older for sure but even my 17 year old plays outside with his siblings in the yard. My 11 year old is out there daily and actively -- my olders (15 and 17) play outside for a purpose, not as much just spending hours on end running around like the littles do.