Hey Michael,
I'm 51 and donated a Kidney last October (2010) just prior to turning 51. I don't think age as much as health is the issue. Beginning in March of 2010, I began working out and lost about 60 pounds by September through working out and eating right. I felt great, but still needed to pass the battery of tests to prove I was healthy enough to donate.
Since donating, I continue to work out, eat right, get yearly physicals and so on. I think it's great you've come to a point in your life where you want to give to someone else in such a selfless way - I encourage you to do so and I hope it works out for you (and your potential recipient) God Bless you for considering it. Good luck!
Scott