Physical attraction is a tingling of the spine. It's basically relying on the animal in all of us. Not that it's inherently a bad or good thing, it's just a fact. I believe "young, dumb and full of cum" is the phrase, that or the misnomer: "you can't fuck a great personality", exclaimed by probably many a young boy who want to "spread their seed". All of these terms speak to a time when hormones were raging and the cup filleth over, when self discipline needs to be taught so that the young minds don't run amok.
But the fact is, you can fuck a great personality. Emotional attraction, intellectual attraction, or even spiritual attraction, often gets overlooked - especially in the younger years. It's something that becomes more important as your grown older.
The act of physical intimacy however is exactly that, intimate. It's a matter of trust, communication, a melding of minds and bodies, something that knows no age. Attraction can be based on several factors, physical attractiveness being only one of them, and can be the way you focus on cultivating your emotions to not prioritize physical attractiveness as much. This is hard in the world we live in, as beauty standards and vanity run rampant.
After all this I'd have to say yes, your grandparents might be fucking - like right now. Give your grandad a cockblock call, and I think his fist might be attracted to your face. At least that never changes.