This spring, aside from training and competing with Piper and Jeddien, I'll also be starting two youngsters.
Big Ben will be 3 in April.   Bea Yewtee will be 3 in June.   Since birth, they have both been handled daily.   They were taught to lead within their first week.   And they were lunging (Parelli Natural Horsemanship style) within the first year.
Last fall, both of the "B" babies had their first lessons carrying a saddle.   A couple of times, Ben also wore a bridle with a nice comfy HS KK "nugget" bit under his halter.   Both horses took their lessons in stride and showed no worries at all.   Now, well into their 3rd year, we are working our way towards having a rider sitting astride.
However, maybe most importantly, their dressage education started at birth.   "Move over" is a verbal command that has been associated with pressure placed generally around where a rider's leg will one day be.   "Move over" is used in the stable, in the aisle when on cross ties, when enforcing personal space, and on the lead when opening gates, passing through, and closing it.   Eventually, the words and accompanying pressure will be used under saddle to encourage sideways movement of the front legs, the hind quarters, and the whole horse.
People often talk about when to "start" a dressage horse.   It's a bit cheeky, but I like to say, "Day 1".   :-)