Here's my 'pseudo-scientific two cent's worth' ......
To a seven year old, a three month summer school break feels like one year. To my seventy one year old self, three years feels like one month. So the older we get, the more time seems to shrink in relative proportion to our time spent.
Einstein's theory of relativity might hold the explanation for all of this, but I never understood most of it anyway. I'm guessing (theoretically of course), that if one could exceed the usual allotment of ~100 years and live way, way longer, time would continue to shrink relative to that longer lifespan.
Live long enough and time could eventually shrink very close to zero or perhaps stand still or disappear altogether... and then what?
Of course none of this makes any practical sense, because it's the kind of stuff I think about when I lay awake at 4 a.m!
I find that when I do my best to savor the (good) days, hours, moments as they come along, they seem to last longer!