They are also part of the first generation of mass computer/internet usage, like us. And they were often slower to adopt a smartphone. So they could very well be going through their "first 5-10 years of internet" experience. The fact that they're old right now might just be a distracting factor.
Or maybe it is indeed a hint that social media is more likely to be used by certain age brackets and we can think of it in waves:
Old people move slowly. They meet fewer new people, they get less enthusiastic about the next big thing, they're less interested in new things, hell, they tend to walk slower, too.
So when a new phenomena springs up, say, a new type of social media, the fast moving teenagers and young adults get in first. The feedback loop between people joining and existing members convincing more people to join blows up the popularity. Once it's popular enough, if it can be easily translated to appeal to older people, the older people will move in. (I say "if", because SnapChat managed to hit the massive growth curve part without ever really getting the old people on-board). The old people being on the site is a downside, so eventually a new novelty springs up to attract young people. The older people, who are slower moving, get left behind.
