Thanks so much for all your replies. Really interesting and helpful to read and fery encouraging that so many have success stories that are very much child-led. I hope that, if they're interested, I can support my kids in pursuing music in a respectful and encouraging way and that their experience and outcome is vastly different from mine.

Starting age seems to vary wildly so I guess I'll just go by their interest and individual temperaments. We have a house full of instruments, so the second they're interested there's plenty of opportunity!