Sooner rather than later

The answer to this question comes in two parts.

First, if the player is not of normal size, they should be fit as soon as they can. A player who is too tall or too short or has any other physical issue should be fit ASAP so that they have the best chance to learn a fundamental swing. This is especially true for juniors. I have spent years watching young kids try and swing their parent’s clubs and develop some bad habits.

If the player is “average” size, then they can play standard clubs for a while. The thing is, a good fitter will have the player try multiple versions and multiple brands of the same club to see what is best for the player. If the player can’t hit the ball semi-consistently in terms of just solid contact, then they won’t be able to tell which club is best for them. The player does NOT need to be hitting straight, or a draw to get fit. They just need a semi repeatable pattern. Once that happens, they can get fit any time.