Why sex should be linked to life expectancy is something of a mystery. Of course, it could be that healthier people are more likely to have more sex and that the findings linking sex to life expectancy are reflecting this, but I think there is more to it. We have seen elsewhere that having good relationships and being positive are linked to longer life expectancies. Maybe sex is a market for good, positive relationships. Of course, there could be a direct health benefit as well: sex triggers all sorts of endorphins and hormones in the body. Maybe these help with healthy aging and increasing life expectancy.
But who really cares about the reason? The simple fact is that having more sex is healthy. Here's a few links to help deal with any age-related sex problems that may come up: