3 years. By then they can count to ten (just about) which is very useful whenever you are tied up and they need something...
"Can you count to ten for mummy, and I'll be finished doing this..." [1, 2, 3, 5, no 4, 5, no I'll start again and get it right, 1, 2, 3, 4, 5, 6, 8, 7, 9...did I do a wrong one? 7, 8, 10!! Well, that's how I do it mummy, is that okay if I do it my way? Everyone can be different can't they mummy?]
Ears bleeding but feeding/nappy/bottles/whatever done!!