As early as possible. I coached it as early as third grade. You need to have at least one kid who dribble with his eyes up and make a pass or they will be running around like maniacs with hair on fire for nothing.

You’ll be shocked at how much basketball basics will make a difference in rec league:

Defensive positioning
Box out for rebounds
Spacing on offense
Screening
Jump stops
Pass to where your teammate is going not to where he was
Etc etc

Have fun!