I think it also depends on what kind of music you listen to. Music that is very "busy" with bass and percussion is more difficult for young kids to pick up, in

I think it also depends on what kind of music you listen to. Music that is very "busy" with bass and percussion is more difficult for young kids to pick up, in my experience. Also the more difficult the vocal performance, the harder it will be to follow. My wife and I listen to a lot of classical and hymns with simple 4-part harmonies. We have 7 kids, most of whom could match the songs pretty well at surprisingly young ages (2 or 3 years old).