Try Adequan or another injectible joint support before you start going into anti-inflammatories. They are very helpful, but I would not start my horse on one before it was absolutely necessary.
Asprin is actually pretty unlikely to be potent enough to help the horse. It is also available without a script, so is less regulated.
If you do go with an NSAID, one of my boarders had great luck with Previcox for her 26 year old mare.
home to 6 overly spoiled, fat, shiny, adorable horses, and 4 cute barn kitties