I normally break in mine at around three (under saddle) I usually have started all the ground work at about 2 1/2 years depending on the mental development of the horse.
My 3 yr old Faith I started ground work at 2 as she is a very smart, pest of a horse. One who gets extremely bored just sitting in the paddock
. She's now broken in and I'm itching to ride her, but have to wait cause I just had a baby.
Everyone has their preferred ages that they like to start horses, but really the horse will let you know the right time. You just have to take in to account their mental and physical developements.