While there is definitely a physiological element to feeling "horny", I think that so much of what fuels your sex drive is psychological (I'm trying to avoid using the word "mental" : ) ).
Given the proper stimulus, I bet you could feel horny more frequently, but at 50, or even younger, depending how active of a sex life you've had, the novelty of sexual encounters can just wear off. It's like, how many times can you beat-off to the same picture or scene from a porno mag or movie? Even with new sexual partners, it can start to feel like, "Been there. Done that. A nap sounds nice!" : )
I agree with the others regarding enjoying what you have, but if you feel the "need", albeit not the "urge", to enjoy sex more often, even though you're not walking around with a permanent woody, then try adding variety and excitement to your sexual encounters (even if it is with the same person). There are plenty of books offering practical advice on how to just that-- safely and effectively.
* Now, go out and f*ck the right thing!
* That's paraphrasing a famous doctor, whose name I dare not speak, lest I face incessant ridicule on this gay social-networking site: