I agree, that 20 yrs old is a perfect age, as long as he's in good health. However, one thing to consider too is that just because a horse is 20 doesn't mean he's bombproof or "kid safe." I know of a horse that spooks and bolts when ridden and she's close to 30. Some older horses can be stubborn, grumpy and herd sour (this is a generalization, but a couple of horses are my barn are like this). Just make sure that you really like this horse's personality. That he is gentle and forgiving and an easy, dependable ride. Hope that makes sense. Good luck!