It really depends on the horse's training and the breed. Warmbloods grow at a much slower rate and their joints take a long time to close, so just the breaking process in itself is usually left until their 4yo year. But in general...
A horse that is steady at the w/t/c is ready to start jumping. Some people do this at 3, it's a little on the young side, although if it's just a cross rail here and there it really isn't hurting anything. And it gets the horse acquainted with jumping.
Some people save jumping u/s until 4, but will free jump them as early as 2 or 3. I think 2 is too young for jumping period. But free jumping *small* things as a 3 yo (depending on the breed) is usually ok. A lot of ponies are started o/f younger.
overall, to really start training a horse o/f, 4 is a good age. If it's a WB, at least 5. Keep the work light and easy, the point is to introduce jumping in a non threatening, enjoyable way to associate them with it. Small single fences or simple lines are really all that I would do in the 4yo year.
"The white horse moved like a dancer, which is not surprising: a horse is a beautiful animal, but it is perhaps most remarkable because it moves as if it always hears music."