Best Age To Wean A Foal!?

[graciiee0x]

I was wondering when the best age to wean a foal is? As ive been told 4 months is okay, and dont leave it longer than 6 months. But yesterday a lady who breeds horses told me she always keeps hers on through the winter until they are 9 months old, and by that time when you take them away they dont care about leaving their mum and the mum doesnt care either, so its like they have weaned them selves
So when is the best age to wean!?
Thank you!!!