I would say that its fine to jump her, but, like everyone else mentioned- stay around 18". Now is the time to teach her not to rush, to stay balanced, how to approach a jump and land correctly, etc. Give her complicated courses with difficult turns or striding as she gets further into her training, and really make her THINK. You can do this all at far under two feet.
A friend of mine has a 14 year old horse who was started as a two year old, started jumping as a early 3 year old, and was already competing at four in 2 foot classes. He is now so artheritic that he's always in pain dispite the acupucture, chiro, and massage therapy that they constantly invest in, and he can no longer be ridden for more than a hack. He was pushed too far too fast and he has paid for it with his health. Now she has a barely four year old little mare that she's doing the same thing with. BEAUTIFUL talented little pony, but she's already being ridden every single day for over an hour, jumping 4-5 days a week at 2 ft to 2'6. Already they're having to have chiropractors out for her. I expect the same fate for the little mare as the gelding.
IMO, a horse is too good of an animal to waste all at once. If you build them up and let them develop naturally, they'll be much better of. Another friend of mine has a 24 year old who is still pretty much in his prime, jumping 3'4 consistently without pain. She didnt jump him until he was four at all, and even then she went slow and allowed him to develope on his own. He didnt jump 3 foot until he was a mid to late 5 year old.