[holyknight]

Yeah, this is a the answer. Our brain only remembers novelty, our firsts years and even decades are plagued by it. But after you are already 5 or 10 years onto your "adult" life you basically do exactly the same each day. 80% of what we do in our daily life as adults is not memorable at all and get completely ignored by our long-term perception. In short, it feels like the time flies because you did only 2 or 3 memorable things in the whole year.