Started slowly and smartly I have never seen a horse go unsound from being started at two. And I've seen a lot of babies started at two, that I have watched grow up and get sold on to new homes. Many, if not all, are still in those homes and competing or trail riding heavily in their 20's. Generally in spring of their two year old year we start getting them used to tack, with light lunging maybe twice a week or so. I will also start leaning over the saddle and sitting on them, maybe getting led around a few times. By early/mid summer they are getting "ridden" a few times a week in the ring, mainly to install steering and brakes. When fall comes, they've been ridden maybe 30-40 light w/t rides, with a lope thrown in down the long side here and there. Then we give them the winter off, maybe with a few bareback rides here and there. Spring of their three year old year I "re-back them", and start some real training. By the end of their three year old year they've been ridden probably 2-3 times a week and been on some short trail rides. Again, they get the winter off. The four year old year we get real serious, and from there the training progresses.
My grandmother used to do things differently, basically her 2-2 1/2 year olds got trail ridden all over the place almost every day, and sold by the time they were 3-4. Most of those horses are still in the same homes, some are in their 20's and still going strong.