The senses dull
The proportion % of the time have an effect on the subconscious
The uncare of the preciousness of "time" when getting older
Remembering = a time-travel vision hence = "instant" feeling, false impression of "yesterday"
Not paying attention to check the time = time pass faster (as a kid, sleeping is a happy thing hence your brain pay attention ONE DAY pass, hence slower passing like checking clock in class)
and
busy/fun = brain is focused. Human turn like a horse with obscured vision. Forgetting "time".
One day pass, likely high% its a boring day
But one day as a kid, is like paradise. Even if school, back home is bliss. Or "friends" in school. Everyday, in school you learn A NEW KNOWLEDGE (though useless /joke)
As grownup, 1 month, probably 3/4 of your time is pure boringness. Combo back to "uncare of the time" and brain no longer use "sleep" for milestone
1min of typing for a kid of 9years, is fun but also a big % of lifespan done on this action
1min for a 25y/o, is less fun but just a mini-mini-fraction of his/her lifespan
TLDR : Its like the "mentioned problem" about immortals having a disproportioned "time feeling"
AND/OR them feeling time suddenly became slow when in love/encountering hero-ine.