If the horse is very well taken care of, I don't see why not, but the only issue with a horse that age having a lot of show mile and experience, is that it may have hoof/leg issues. If you do a pre purchase exam, I would MOST DEFINITELY xray his legs. II have heard and seen a lot of people make that mistake.
I would say that an 18yr old, if it's not burnt out from competing & healthy/sound, then sure, he sounds great!
I agree with the above posters - if the conformation, condition, prior care, and current training for what you want are there, go for it. BUT also be sure to do a pre purchase exam WITH xrays for a horse that age. You never know what is going on the those legs, and the current owner might not either.
I would also say to make sure the price is right. I would not personally pay as much for a very experienced 18 year old horse as I would for a slightly less experienced 5-8 year old horse with the same lines and conformation...but that is me (I like the training part and younger horses).