I am really glad to see so many know the dangers of lungeing, it is unnatural for a horse to go in tight circles for long periods (tho I see it done enough
) and it not only does no good for exercise or training, but can mess a horse up, straining their legs and building up the wrong muscles. Lotsa studies.
However, if NOT overused and abused, it can be ok, I used to lunge myself back in the day, but I have found round penning to be far more productive as far as training (I can get my horse to do anything someone lungeing a horse can do, and without rope or halter (or whip in my case!)) and as far as exercise. If you do, I agree not to start earlier than 3.