I think it is a few years at least! but as they get a bit older they do tend to ask less about such ridiculous things. I assume they learn how to use it properly through trying it out alot and it seems alot is ALOT!

I know at 3.5 you are meant to reverse it and say thing like "Why do you think have to put on a seatbelt etc"

One of DD's friends says - "Stop saying Why! - I don't like Why" someone must say it to her and so she is repeating, but I really don't like it - although it is annoying to think of answers all the time - think is wrong to tell them to stop saying it.

I am a bit of a 'Why' adult though - so maybe that is why doesn't bother me. Is great to question things after all.