Horses at that age often do start getting a little stiff and can benefit from joint supplements. I've heard really good things about Cosequin ASU, but it's pretty pricey. The SmartFlex line of supplements are also pretty good, and have some lower price options.
^what beau said. I know that with my mare we started her on msm and that helped her a lot. When she started getting more excersize and was out more then we stopped it as it wasn't needed. We only started her on it again now that she's older recently. (It was also pretty cheap compared to some other stuff... lol)