The obvious non-evolutionary answer is that it's still enjoyable. But if we're just talking evolution, the question becomes "why is sex after reproductive age still enjoyable since it has no evolutionary purpose?", that would be the clearer question. The answer is that a cessation of libido didn't evolve. There's no selection pressure for it to. Our sex hormones are still quite useful in other ways and changing their functioning so drastically would be costly and pointless, not something that natural selection works towards.

I don't think it's just about having fun, as a person married for almost 20 years, it's about closeness, intimacy, and showing love and caring for each other. I think people, especially in long term relationships want that no matter their age.

Edit: grammar

evolution doesn't care about anything after breeding age. the programming carries on. sex = fun, age doesn't change that.

well, men can father a child until the moment they die. even after, technically speaking (gross). and frankly, womens sex drive DOES diminish significantly after menopause.

i think that older humans probably just have sex because it still feels good.

We have sex 'cause we know what's actually attractive - and you don't, not yet.

'Cause most of what you think is "hot", we know isn't hot at all - from sad experience over a lifetime, friend.

Shallow good looks and a venal personality turn us off, not on. Unlike you, who haven't been betrayed by that succubus (or satyr) yet. Come talk to us soon, when your grief seems unfathomable and you can't understand why any young person would have sex, if it leads to so much pain. We won't mind your tears. We know there are many better nights to come in your life, when you're old enough to appreciate them.

We love to have sex because we that know youthful narcissism is far uglier than a gentle wrinkle forming by fifty years of kindly smiling, and we also know that ignorance of the self makes you far more unattractive than being absent one or two of your natural teeth.

Poor youth: what you see in a face of the opposite sex only what a photo of that person when they are unconscious would reveal: symmetric bone structure, skin tone, and that's about it. We see who they really are in the details of how they use every part of their face every split-second. Gorgeous. Which means we old folks get to have sex with actual people, while you're stuck with humping a very vague idea of what a person of the opposite sex might or might not be like, but actually isn't like at all.

And the icing on the cake is, we still remember the vivid illusions of our youth, and can savor our memories of those youthful fireworks, too, without having to suffer the consequences of that craziness anymore.

Because it's fun and it feels fantastic. Duh!

It may well have evolutionary advantage. It keeps the grandparents pair-bonded so they are better able to help rear their grandchildren, so helping their genes to replicate successfully. Evolution is driven by genes, not their carriers.

But maybe the question is why fertility in women ceases in mid-life. Again, this is because it leads to greater success. Human children need a prolonged period of dependence and, because of evolutionary changes to the birth canal and the size of human babies, giving birth before the advent of modern obstetrics, was almost uniquely dangerous for human females.

Children whose mothers died would have been far less likely to reproduce themselves. This would have provided evolutionary pressure for women to reduce the number of children then have and to spend the time rearing their fewer children so ensuring greater ultimate success.

Retaining sexual activity would have retained the mutually dependent relationship between man and woman. Besides, what evolutionary advantage would there be in losing it? Change doesn't occur unless there is something to be gained by it.

If you want to learn about the evolutionary history of human sexuality, I highly recommend reading Sex at Dawn.

I didn't even need a book to learn this, but I did hear it in a movie. Everything a man does, he does to get laid. EVERYTHING. Now I could be far more PC about that and put it in scientific terms, but why bother? And yes that old guy flirting with the nurse...he knows its a 1/100000000 chance but he's still giving it a go :)...Not sexist, female versions do apply. But as has been pointed out men can procreate until the day they die....

Now this might be almost a no brainer. Sex=fun. If you're not having fun, why are you still living? All this paleo way of life and healthy eating means nothing if you aren't enjoying every moment to the fullest. Sex is one of those nuggets of fun, whether they are one night stands or with your wife of 50 yrs. May sound preachy but live every moment like its your last.

Because someone needs to teach you silly, know it all young people how to do it...

I think there are two kinds of sex drive:

One is the drive to reproduce; I associate that with feeling horny, and it's not really very picky. (Are Porn & Vibrators the industrial fast-food of sex?)

The other is a soulful desire to connect in a uniquely intimate and deep way with another person.

I only have 8 more years of practice before I get to the coveted SEXagenarian title.

Social bonding.

How old is "old"? I mean... for women, past mid 40s gets real dicey for making paleo babby. Some women lose the edge they had as a youngster and 'being hot' takes a back seat to other priorities in life. It's sad to see that happen. But let me tell you, a lot of women still have the goooooooods in their late 40's and even much older, especially if they take care of themselves as well as they are able to.

Guys? Well... guys are like the energizer bunny. Men can reproduce till the very end (and believe me.. some do. I've just recently heard some pretty crazy stories about that). Even old cranks in their late 70s can get it going pretty good. Some guys get a big belly and fear baldness and stop grooming and just let themselves go. But if they get sick and tired of being out of shape, they hit the gym, and look better in their later years than they did in their mid 30s. They get more suave and sophisticated as they get into their 50s and 60s. That's why many young women dig older guys. And the consensus seems to be that the males don't really put up much of an argument with that part of the equation. Although the older cougars usually want to claw the girls' pretty little eyes out. lol

I read in a book once (Think and Grow Rich, by Napoleon Hill), that, at least for a time, sex drive is behind almost EVERYTHING. Every single significant behavior or effort. Without that drive lurking in the background, there would be almost no motivation to do anything in life.

Old people have sex because sex makes the world go round, and not just for reproductive purposes.

I just skimmed through the answers, but nobody seems to have asked the obvious question:

-what constitutes "old age?"

We have a longer average life span now then we ever did in history, despite the fact that most of us are shoving absolute crap down the pie hole.

So, just because most of make it past menopause and such, doesn't mean that most of our paleolithic ancestors did.

More curious still is why nature would have selected for the end of fertility in women, but not men and how nature would have selected for this if those women were no longer reproducing. Perhaps it was an accidental mutation that occurred along with another, more beneficial one...

I think it's cultural. It's all around us that we should be having hot sex as much as possible. If it were socially acceptable we probably would just let it fade away. Instead we buy Viagra and try to keep it going as long as possible.

Because the more I have the longer I will live.........its third grade math.