Oh, whatever. 2 years from now, they cut another album, and go on world tour, again. Age really isn't a factor, it's a matter of whether you still love the music, if you like to sing, but no one would ask them to sing another note, if they don't honestly want to.
'Old fart' story: Once Upon A Time, when I was a little boy, I got to go the Berliner Philharmonic, one of the strangest-looking buildings I've ever seen in my life. We climbed up, up, and up, to our nosebleed seats, high above the stage. Everyone settled into their seats, and this little old man came out onto the stage, and people applauded, and he sat down, and started playing his guitar. He played, and played, and people clapped and clapped, and he played alone. For about 3 hours. And, each time he tried to say good night, the audience kept saying, 'zugabe', 'zugabe', encore, encore, and clapping together in time. And, he would come out, and play again. And again. Finally, he could play no more, and everyone left. But, the point here is that the man was well into his 80's, loved to play, and people loved to hear him play. His name was Andres Segovia. He has since passed, but there's probably still people listening to a Segovia record, somewhere.
I remember ABBA from when I was 8, or so, about the same time that I saw that Segovia-guy. I played their record, over and over again. What a great band, what a great sound. It's hard to say what, exactly, is the 'backbone' of their sound, because the combination and contribution of each member of the quartet was the 'backbone'. The vocals against the instruments gave the signature ABBA sound. Agnetha and Frida sang harmony, Bjorn and Benny brought keyboard and rhythm, and the final product was nothing less than golden. Can't beat that with a stick, so to speak. I'm no music expert, but I've listened to a lot of different stuff over the years, and theirs is a sound that stays with me, and also kind of serves as a benchmark.
To some, ABBA might be old hat, but sometimes, that old hat fits the best.