When do boys (and girls actually) learn to do up their flies?

[KatyN]

My son is just 4. He can undo the button or popper on his trousers but has no hope of doing them up again.
Nursery suggested he wear joggers more often but in an effort to have some standards (!!) I have just trawled the shops looking for pull up nice trousers!
Just wondering how long We have to go!
Thanks kx