My Galahad was 22 when he came to me for retirement and I spent ages very carefully rebuilding him back up from a harsh environment. The best supplement I found to keep his joints mobile was 1/2 Tablespoon of powdered myrrh in his buckets twice daily.
At one point the vet declared him ‘toothless’ too, so I was soaking his mixture of soaked beetpulp and Strategy 3 xs a day (he was prone to impaction colics because from the time he was 3 years old, Galahad never drank enough water) plus all the grass hay he could eat (good leafy brome). The myrrh proved critical to his mobility. When I ran out, I noticed an immediate stiffness in his hocks, only to be returned to free motion when I put it back in his rations.
I did add a lot of things to his rations depending on what he needed, but L-Lysine was helpful to keep him in top form.
I lost him in July, 2013 to a colic that the vet described as his body shutting down, but the blood panels taken the day before, the vet said he was looking at the blood panels from a 5 year old horse, so clearly we were doing right by him.
Hang in there with your gal, oldies are worth the extra work.