A few more details besides his age would be helpful.
If you haven't already done so, a good lameness eval with an equine vet is the right place to start. An aged horse is likely to have some sort of arthritis problem. It's good to know what you are dealing with so you can tailor a program to your horse.
For example, if your horse has bone spur on his hock, you can feed all the supplements that you want, but it probably isn't going to help at all. If you want to continue riding, you'll need to do something further such as injections or Previcoxx to manage the discomfort, in this example.
Yes, supplements can be very helpful, but if you don't know what you are trying to treat, you're really just guessing
on where to spend your money (and potentially wasting it).
Also, it's really a catch 22 on how much feed-through supplements actually make it to the joints. There's conflicting research. With that said, I still feed supplements to my boys (either Platinum Performance CJ or T.H.E. Muscle Mass custom blend with joint support). But you can also consider other therapies such as Adequan or Pentosan to help maintain your horse's joints.