I have a 24yr old horse with very bad arthritis in his front knees, obviously I don't ride him
I use Grand Flex Senior, it is rather expensive but trust me it works very well! He is mobile because of it and I also own a Pulse blanket which I use on him too, these are very expensive as well but you don't have to buy one, you can often find someone in your area who can give your horse a treatment, they are very affective.
Here are some links to my suggested ideas: Grand Meadows Grand Flex Senior Home. Pulsed Electro-Magnetic Field Therapy - Pulse Solutions - Christchurch New Zealand.
(p.s. There not just in New Zealand, they are all over the world)
I highly recommend these! Good Luck!