Well I start introducing my horse to jumps basically as soon as I broke her in when she was 4 or 4 & 1/2. However, I must say that we never jumped more than 45cms or so for about a year, mainly because I don't really like jumping very high myself, not because she couldn't have managed it. Now, she really enjoys jumping and is quite good at it. I don't see a problem with starting a 4 year old to jumping provided you don't over do it. It can depend on the horse's breed and temperament a bit also. If he's really hot headed, I wouldn't start jumping until your really sure he's settled enough to listen to you when jumping.
I don't think there is an "age" depends on the horse, their balance and learning ability.
I say you can start jumping as part of the training process at age 4... start off with lots of trotting poles and small X's, many times jumping can help horses with their balance. Just do everything small advances and keep it positive, you want them to enjoy it not get too much too fast. Horses do keep growing till they are 5- so that's when I would start really presenting courses and uping the height to their potential.
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.