Who says grandparent's aren't having sex? I don't really go around asking my grandparents what their doing...but I'm sure there are a bunch of old people out there still doing the nasty. I don't think you're hormones disappear after a certain age.
And besides, if everyone just had sex to have children the earth would be so overpopulated. And we don't need anymore people...if anything I think people need to stop having kids.