She is about 20 (by her teeth - her papers are somewhere in the trainer's office files) and she's slowed down a step or two. The Red Cell really helps her energy level, she does a good deal of work - our trainer uses her for lessons when I'm not riding her, and of course during the fall and winter she hunts with me, during the spring and summer she's doing combined training, not to mention the occasional hunter show when I get nostalgic - so she's a busy little girl.
She's also getting the glucosamine/chondroitin for her occasional stiffness. The vet can't really pin down anything substantial, she's just a little gimpy on cold mornings until she gets warmed up (I am exactly the same way, so I know how she feels!)