The rule does actually allow you to show a pony at 18 because of the way the birthday is calced.
And yes, if you had a pony that had not been measured and didn't have a card, you could show it in the horse divisions but you'd be showing at a considerable disadvantage.
It may help to remember that the Hunter Divisions were originally designed as a progression - Short Stirrup to Pony Hunters to Children's Hunters to Junior Hunters and Equitation, then moving to either Jumpers or the Adult Hunter Divisions. Children weren't expected to stay on ponies much past 12 or 13, back in the day. Pony Equitation is a more recent innovation, if you were skilled enough to ride a real equitation test, you were expected to be on a horse.
The division structure frequently lags behind current reality. Several pros have told me there's no such thing as Children's Hunters anymore. Kids stay on ponies longer, a lot of them until they age off, or go straight to the Junior's and the Eq.
Also, it you wanted to show a pony in a pony division, many people will think it's unfair for an adult to compete against the kids. You could set up a separate division for adults on ponies, but I suspect the division wouldn't fill.
So, as a 5' adult in rated shows or shows run according to USEF rules, your best bet would be to show a small barreled horse that can make the striding in the AAs or AOs. There are horses out there that you should feel comfortable on. If you look around you at a hunter show, you'll see lots of examples.
If you really are only going to feel comfortable on a pony, your options are unrated or schooling shows that don't follow USEF rules, the schooling jumper divisions were adults are allowed to show ponies, or eventing.
Last edited by maura; 07-16-2012 at 10:56 AM.