What is the max age for equitation finals?
17 in horse show year, December 1 of the calendar year.
Except for the USEF Talent Search (formerly the USET), which includes riders up to the show age of 21.
As I believe OP is now 18+, she is eligible to compete for the Ariat National Adult Medal final.
If OP is an adult, the various regional finals (New England, MidAtlantics) also have Adult division finals. Very nice for those of us who didn’t have the skill/resources to qualify as a junior for anything, or those who like to continue doing the Eq stuff.