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.