Turning 40 this year. I actually have pins in my leg form a hip break a while back (fluke, slipped on some ice, and no my balance is not generally horrible...)
About a month after I started playing disc golf (playing every single day for at least 2 hours mind you...) I started developing a lot of pain in my leg. I was worried that was it for me because of the surgery but the doc gave me some stretches to do and it relieved the pain and now my injured leg is actually way stronger than it has ever been since the surgery thanks to disc golf.
I've been playing (incessantly) less than a year and have taken a couple lessons from a local pro, but have managed to get my distance up to the 400' range.
There's a lot of technique plateau in the 280-400 range so I wouldnt count your distance gains done. As another poster pointed out though, as we get older good form is not as much about distance as making sure that you arent injuring yourself or causing more wear on your bits than you should.
That said, keep practicing form. Clean it up, watch Feldbergs "the jewel" video to help on angle and run up, figure out how to accelerate your hips and shoulders at the right point in your turn. Your planted foot is your fulcrum - you're basically trying to run up, plant, and create a whip around that fulcrum. Relax and let yourself whip.
But the biggest key for me in getting past the 350+ plateau and also ironically in reducing the wear on my body has been learning to follow through all the way. If there's one thing I would preach about saving your body its that.
and finally - Scott Papa talks about getting older and being a pro on the DG circuit. Check out his bio on the Discraft site - I found it really informative and helpful. One thing he mentions is cross training and stretching becomes more critical as we get older since we dont have that awesome just-because-you-are-a-young-male core strength and flexibility.
edit : its Mark Ellis that talks about getting old, not Scott. Sorry guys.
http://www.discraft.com/team_ellis.html