10 is not too old! I didn't start till I was 11 and played into my 30s - had a 10 year break then came back to it. Am now back playing county team league and am representing my county next month. My current coach didn't start until he was 15 and has coached very high ranking players
Not too old. He'll catch up quickly particularly if he's keen to have a go. All sports seem to offer programmes to catch them when they're young but in reality while they can learn useful skills most kids don't have the strength/stamina until 8 or 9.
I don't see how he can be too old at 10. Of course when other dc have started very young, they will have timing and ball control which he will have to acquire but I am sure he will not be the only dc they have starting at 10 or indeed much later.
10 is not too old to take up anything! Why don't you have a few lessons together to start him off then put him into mini squash? Is he generally sporty/does he have good hand/eye coordination? If so he will probably soon catch up with the others.
DS has announced an interest in squash, which I would like to encourage. I don't play myself and though I am willing to learn, I suspect he might have more fun with some proper coaching. Have looked on the website of the local leisure centre and racquets club, and it looks as though "Mini Squash" is something that starts at age 5! Is 10 too old? We just want him to have a bit of fun and get some exercise, but I know he will be put off if everyone else has been at it for years.