The most important is to get the connivence and partnership with the horses. You must always ask yourself: "If I were the horse would I be happy to have me as a rider?" Your horses must be happy to see you put the saddle on and go for a ride. You have to make them enjoy the ride and get them happy to learn.
I don't do very young horses anymore but I always did before I moved to the US and used to start initiation courses when they were 4yo. Some horses are late though from either still growing or still "green" mentally and I would wait one more year with those.