Not knowing how your 4-H program is set up there, my advice may or may not be an option in your area. Most 4-H programs allow you to exhibit more than one animal. They generally have their halter classes split into age groups, here your 18 yo would be in the Aged Mares 16 & over and the younger would fall into the 5-9 yo mares. If it is set up similarly where you are, I'd say take them both. It would be more experience gained for both yourself & your mares.
If they just have a mare halter, no age, I'd go with the younger mare as she will have more chance to be competitive than the mare with more age would against younger stock. Get them both legged up & fit, hill work if you have access to hills is a great way to get them fit & build muscle & lots of trotting to tuck up your buckskin mare's tummy ;)