How old you need to be to hold the strings down? I'd start with piano first, and then branch out, but that's just me. I started learning piano when I was old enough to press notes down, went through a brief percussion stage, and finally recognized the call of the bass. I don't remember a time when I wasn't playing music, any better than I remember a time when I couldn't talk. There's no "too early", but you shouldn't have to push or force the issue either. It's very much like learning to talk--there will be an early period where you'll just be making noise.
In some cases, the child will never pass the "making noise" stage, or will revert to it at a later date and join some sort of avant-thrash band.
When is the right time to start your child on a lifetime addiction to making music, chasing tone, and buying gear? It's like asking when is the right time to introduce your child to crack. I love my parents, and have forgiven them, but I can't seem to shake the addiction. I'd write more, but I have to go play cover tunes for drunk people. Luck!
(Searches for "Merge Posts button")
Woodwinds, and those plastic recorders. Have been the first music instrument for little kids since forever in allmost all cultures.
I always assumed this was because plastic recorders are very very very very very cheap.
"Until they feel like it" is the right age. When surrounded by instruments, at some point kids will want to pick one up.
For improv, sure. For basic theory, well, ask yourself this. If you waited until your children "felt like" learning to read... I'm just saying that the world doesn't need another drummer who can't read music.