Entertaining snarkiness aside, the right time is when your kid is ready. You don't want to keep him at a level where he is no longer developing, regardless of how much you like the coach. Similarly, you don't want him to be on a team that is at a level of play that he isn't ready for.
For one kid, it's age 7, for another, 14.
It's all about coaching ! Just left an elite EHF team because of horrible coaching.
Go where your kid can develop otherwise waste of time !