Newborn - Touch them all over, make sure they trust people.
1month - 1.5 - Again, make sure they are fine with leading, touching them, and brushing (obviously not a big old hard brush).
1.5 years - 2.5 years - Get them used to the above, but take them out almost everyday, turn them out in the arena, get them used to more "scary" things. Maybe even lead them on the trail.
2.5 years - Start lunging, and putting the saddle on.
3 years - get used to human weight, and when ready, get on and walk for a while. And, start lunging.
3.5 years - Start actually walk, trot and canter.
4 years - Do ground poles, get them balanced, maybe even enter a few dressage/flat (if you are focusing on this discipline) classes and take them to new places.
5 years & up - Jumping, and get them into the world a lot more.
I know I'd start my horses a lot later than most, but that's my opinion. I'd let their joints and muscles settle.