I pretty much agree with the previous posters.
Of course, I got my Arab mare when she was 23. She had been retired since she was 11 or 12 for going over backwards on her rider and, obviously, had some pretty serious issues undersaddle. But nobody told me any of that^ until after I had agreed to take her [a mutual friend was acting as the sort of "broker" between my mare's then-owner and myself...no one told the mutual friend these things until after all paperwork had been signed].
ANYWAY, long story short, Lacey is now 28 and basically one of the most dead-broke but sensitive horses I have ever had the pleasure of working with. She is genuinely a pleasure now. There's never any doubt in my mind that she is a 100% safe horse - I even use her to teach little kids how to ride!
It took a solid 3 years for me to get her trained the way I wanted, but I had never finished up a horse before or worked with one that had so many issues. An experienced trainer could have gotten her fixed up in no time.
She's also a kickbutt trail horse. She'll attempt anything I ask her to try! She had minimal amounts of trail experience, from what I've heard, in her previous home but not a whole lot.
Anyway, 20 is nothing, especially for an Arab. Of course, you do need to be prepared to be this guy's last home since the market for older horses is small.
And older horses do have less...I guess energy. I mean, my girl will keep going for as long as I ask her to but she gets TIRED and it shows after a bit. The 9 year old Arab I work with at work could go all day, and then some, without showing any signs of fatigue. I guess the thing is that my older girl is more willing than her body is able, she just doesn't have the energy reserves she had when she was younger. She's still totally up for 3-4 hour meandering trail rides with breaks, but she'll be pooped by the end.
The other thing: older horses are less able to be "molded" into whatever you want them to be, like you can mold younger horses - if he doesn't want to be a dressage horse, you are going to be fighting a HARD uphill battle to make him be one. He may love dressage and you two may be a perfect pair...but be prepared for the "what if he doesn't like ____". That's why my girl is a trail horse - I wanted to do arena "stuff" like jumping, dressage, etc...and she didn't. Guess who "won"? haha
Of course, now I absolutely love the trails and live for hitting them...but that can be a problem.
Older horses are great. I just love them. If you have the space and resources to be able to keep this guy for the rest of his days, I say go for it. I don't think you'll be sorry.
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.