Hi! I have a few different thoughts on this - one more abstract and one a bit more concrete. I'll start with the former.
Personally, I'm not sure there's a set age that comes to mind for me. Maybe see what your child naturally likes to do? I've heard of plenty of drummers/percussionists who banged on pots/hit things with hands/etc when they were really little - look for signs like this, because when you start them doing something they already have an inkling to do, there is more incentive for them to do it and get better. (Now, of course, don't expect this to happen, but I'd say just to always pay attention. They're little kids after all!). All in all, if they show a special interest in anything - nurture it! Regardless of age.
Sorry if that is a different take on your question! However I think it is a viewpoint that can easily be missed. If I were to be a bit more concrete, perhaps 7 to 9 years of age, especially if they're not really showing any interest in anything in particular. Again, if they ARE showing interest, really nurture that.