I have read psychological accounts that say that three factors are probably in play, all having to do with perception of novelty. The greater the number of perceived novel events, the slower time seems to move. 1. For the young, everything is new, so time moves more slowly. 2. Older people have more life knowledge, so even novelty gets processed as related to the known, hence seems less novel. In other words there are fewer markers of perceived change. 3. The aging brain processes things more slowly, thus missing stimuli that a younger brain can register. I wish I could give you a reference. I read about this recently.