My horse came up with a bum knee around age 17 or 18. I chose inject to her for the first time (mine and hers). She went sound immediately afterwards. Probably the best money I ever spent with a vet. It's been several years and she favors the knee a bit now (but she has other bigger problems) and I feed her SmartFlex Senior pellets. It's done well for her.
Your horse is "at the age" when things start to happen. Depending on the severity ( vet DX) you can try several things. My big girl ,at 18 ,is on Adequan. We have it down to every 6-8 weeks after the initial loading doses. She has issues with hocks and high ringbone. That said, I like to start with the minimal expense and see what works. Corta'flx is very good, Smartpak has some good joint supps, as does Horsetech. Th elast two will give you samples. MSM sometimes is all you might need. Be prepared to give whatever you try a fair trial period,maybe 1-2 months. I agree with lots of turnout and try to keep them moving. Good luck.