My son is only 2, but we decided to get him into "school" last winter.
When my wife was interviewing these places, the choice was between "learning" environments and "social" environments.
We chose the social environment. There's plenty of time for him to learn numbers, letters (he's only 2), but it's more important now for him to to learn to take turns, run around with other kids, bang on the drums and sing when the weekly musician comes in, etc...