If you know the horse, and are convinced that it has some pep left, then it's certainly an interesting prospect. If you can get it for a good price, and have it properly vetted (as always) in advance, I see no reason not to go for it. And as others have said, and older horse knows the world well, and will be a valuable partner. There is no substitute for experience.
We live in a society of planned obsolescence, and are more and more conditioned to believe that something "older" is weak, frail, and valueless. I purchased my first horse recently, at 15, and I remember asking the vet if he'd have enough pep and energy for the forseeable future. Boy, do I feel like an a**. I think the horse heard me, because the first time I opened him up, across the spine of a mountain...well...my doubts are thoroughly resolved.
Now 15 isn't 20, but the point stands. 20 has experience, and plenty of fire left in most cases to give you years of, well, the best thing ever.