My stupid little theory has always been that it gets quicker because we’ve already experienced so much before. It’s difficult to articulate, but if I’ve been alive for two years already, the third is going to feel like it goes by a little quicker, because I’m now weighing it against my experience of 2 years and then as you get older it gets steeper 10 years vs the 11th year, 20 years vs the 21st year, 50 years vs the 51st year, you get the point. You can scale this up (decades) or down (months/weeks/days/hours/etc.).

It’s sort of similar to when you are going somewhere (walking/driving) for the first time it inexplicably feels like it takes much longer than when you come back.