A year ago I bought a mare and gelding qh pair from an auction sight unseen. I got a killer deal on them, $400 for the duo. They were advertised as 15 years old and bombproof trail horses. The gelding is 15 and a bombproof trail horse. The mare, upon vet inspection, turned out to be estimated 21-24 years old. Despite this, she is in perfect health (no arthritis, teeth are in great condition, etc) and is a jam-up riding horse, although she is a bit on the hot side. I had wanted to train them for barrels, but due to the mares age and the geldings demeanor (he is a deadhead lol), I ended up purchasing an 8 year old AQHA mare to pattern. 
I took my older gal out for a ride today. I had the barrels set up and decided to breeze her through them just to see what she would do. I trotted her to the first barrel, and as soon as we turned it she leapt up and took off. I pulled her back down to a controlled lope for the remainder of the pattern. After working her a little more, I came back to the barrels and asked for some speed, and she definitely gave it, it's obvious she's been a barrel horse at some point in her life. 
I would love to haul her to a few local races. I'm currently exhibitioning my 8 year old, but this mare feels like she could place in the 3D/4D if she's legged back up. 
Is this crazy considering her age? Does anyone run any seniors?

Not at all. 

Horses are like humans. 

Some 70 year old humans are in a lot better physical shape than a lot of 30 year old humans. 

Yes! My 90 year and 80 year old parents are in better shape than most 50 year olds. Dad still cuts and rakes hay, works on the tractors and trucks, helps work cattle etc. My mom watches my grandson several days a week for up to 6 hours a day. 

Some horses just age better than others. Just be sure to take care of her needs a bit more throughly: liniment, longer warm-ups, ice legs. I think some of the older horses are written off too early simply because of age.

By inspection I assume you mean a vet inspected and she's good to go? I don't see why she can't run if the vet cleared her. I know a 26 year old horse that qualified for Best of the Best and you have to place in the top 2 or 3 in the 1D at certain qualifier shows!

I have a 27yo gelding that I'm thinking will run selectively this year.  He did fine last year and has wintered pretty well.   I will start trying to leg him up soon to see how he does.   He wants to go and is still competitive in the lower D's.  He gets so sad when he's not being used. 

That's another thing! I don't think some people understand that some older ones don't want to be retired, if they're still cleared by the vet to go why not take them sometimes so they still feel like they have a job?