One of the key findings recently about how we experience time is that time seems to speed up as we get older because we have more experiential knowledge. Things are more novel when we're younger so they make more lasting, striking impressions. As you settle into a familiar routine, with school or work or with social relationships (romantic, platonic, familial, so on), the novelty fades and things stand out less, so time seems to pass by more quickly.
It's my pet theory that one of the unusual benefits of having a child is that you get a little bit of the novelty of experience that they can't escape. The whole world is new to them, and they force you to re-examine your surroundings, your circumstances, your self, in addition to adding to experiences because you can't (or at least shouldn't) escape the responsibility of rearing your child.
I'm in no way saying that everyone needs to have a child to get the most out of life or that a life is incomplete without having a child, but I do think it's at least in small part an evolutionary advantage that homo sapiens, and very likely other relatively long-lived and intelligent species, adapted so as to have more significant experiences through longer periods of life. Especially in modern times, with the lifespan of a human being outstripping by decades what it used to be in ancient times.