Generally speaking horses age 3:1 so if she's 12 horse years, that makes her about 36 in people years.
That's about the age of the vet since it takes at least six years to get thru vet school and then do an internship. He needs a refresher course regarding who's old and who isn't
She is far from senior but sometimes younger horses do need senior feed if they have some sort of issues.
If your mare is not over weight but a fairly easy keeper and is not being worked hard (as in several days to every day of the week for an hour or more), she would probably do well on nothing more than a ration balancer and plain old grass hay.
If she's in foal, that's a different story and I digress to the experienced folks that are currently caring for mares in foal.
My four geldings range from 16 to 25+ years. Their diet is basically all pasture & hay with a good vit/min/pre-probiotic, rice bran and Omega-3 Horseshine.
Two have metabolic issues so they get herbs and minerals to keep their insulin under control