One very simple explanation I’ve read is that time perception (as time itself) is relative. Time goes slower when you are a child because your perception of time is relative to the totality of time you’ve experienced.
So for instance, when you go from 1 to 2yrs old, it’s double the amount of your entire life, but from 80 to 81 it’s just a small fraction.
Not sure if this has been proven or is even possible to prove.