Might also be related to brain activity. Young brains learn more in any given situation and time period, old brains just kinda work with what they already know, going on cruise control. So perhaps a young brain really has a lot more stuff going on at the same time, absorbing more of reality, everything being new and fascinating, constantly "in the moment" and taking note of everything, etc? So instead of time being different, maybe young people have more impressions about their surroundings for any given time period - more observations, more new interesting things learned, more curiosities and mysteries, etc. Older people are on auto-pilot, very little is really new or fascinating, very few reflective moments, everything just starts being a big streamlined auto-pilot with some exceptions here and there. So less stuff seems to happen for older people, which may make it seem like time just flies by, if we were to measure time by the number of observations/lessons/experiences that we consciously registered.
The C's did say that at the moment of transition to 4th density, for a very brief instant you know everything there is to know, and that moment may last seemingly forever. Maybe the reason that we perceive eternity in an infinitesimally tiny moment is because our awareness of reality is extremely expanded for that moment (they called it becoming illuminated), so there is a lot more IN that moment that we can now observe, which we couldn't normally. Similarly, maybe kids just are aware of more things in any given moment or something, and old folk are not. Old folk may know more stuff, but as for being present and aware in the moment and paying attention to all the little things that are now "boring" to them, they may not be doing that. When was the last time you watched raindrops falling on the ground or into puddles with fascination, for example? Or analyzed with great curiosity the texture and veins of a leaf (or made a leaf skeleton by ripping a leaf in such a way as to leave the veiny skeleton thing intact), or how wind sounds when someone almost closes the door but leaves just enough space for a whooshing/whistling sound, etc.
Having said that, I wonder how time feels for people with attention deficit disorder of some sort?