Apologies for the late posting, but I just thought I’d chime in!
Six is my current minimum age. Any younger and I’ve found their attention tends to waiver after a short amount of time and you don’t gain any major ground in a lesson.
Saying that though, if you’ve a way to keep a child engaged with the lesson then there’s no reason that three, or even two, would be too young – I’d just make sure things are fun and colourful!