I admire people that don't automatically jump on their horse at 2. If they have the time and patience, I see nothing wrong with holding off. Especially with a jumping prospect. Hard work on those legs and joints.
The mare is reasonable $ for her age, breeding, etc. The ad says MAY trade - they are specific for an AQHA or APHA with color. This mare is an appendix.
As far as needing to be 16+ in the HUS ring, wrong. The horse needs to have abilility vs size.
I looked at the ad again, but it says for her breed is AQHA American Quarter Horse, and that she's registered with the AQHA.. no appendix there. Unless you can register an appendix with the AQHA