M.O.
It STILL works with mine, and he's 7.5. As far as I know, it works forever. For a lifetime. It does have its drawbacks, though, because if you make everything about winning, it's harder for the child to accept losing, and this is something only children (or at least MY only child) struggle with anyway.
A variant that I like slightly better is "how fast can you....." Now that mine is 7 and highly technological, we time all kinds of things, getting dressed, finding shoes and jacket, etc., with the stopwatch function on my phone.