I'm an equine podiatrist, so 'do' horses' feet for a living!
I would agree with starting to work on a foal's feet as soon as possible. Some foals wear their feet on their own, but some DO need rasping and balancing from an early age, depending on the surface they're kept on, and their conformation, and because of their soft bones and rapid growth rate, if they have issues with their feet at a young age it can have detrimental consequences for later on.
I have a 9 month old foal who hasn't had her feet trimmed at all. She had been raised on the Welsh hills so wore them down herself fairly well, and I'm lucky in that she has good conformation, but of course she's never even been taught how to pick her feet up, let alone be taught to stand for them to be trimmed. The earlier you start to teach them how to have their feet trimmed, the better, in my opinion.