A lot of people start their horses at two because they have shows (or classes) that are age specific, so they start at 2 so they can do the 3 year old class at the big show the next year. Personal preference, I won't start a horse until they are 4, as they still aren't really done maturing until then, but I will do a lot of ground work, before then, making sure that they are easy to handle, and responsive on the ground. Personally for me, I have no need or desire to hurry a horse along, and start them at two, though I know of plenty of horses who were started at two, and are still doing fine years later. A lot of it I think comes down to personal preference, as long as you are willing to go slowly. I'd much rather have a horse who was started lightly at 2, then a horse who at 4 or 5 is being started too quickly, is ending up with holes in the training, and a bad attitude because they are being pushed too much.