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

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.