My Lacey girl is 25, 26 in the spring, and she certainly doesn't look or act her age. I suppose, if you looked hard and had a picture of her not moving, you could probably see the age. But if you saw a picture of her moving or just saw her irl, you'd have no idea.
She does get stiff if I only work with her once a week so I try to get her moving 3-4 times a week and that seems to keep her younger feeling. For instance, on Monday we went for a ride that ended up being a couple miles (maybe 3-4 round trip) long over rather steep terrain and she was sweaty but not too pooped when we got home and the next day she was raring to go again!
17 is young! I wish my girl was 17! Haha
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.