A friend of mine from some time back chatted about this once. His take was that as you get older, your "mental models" grow and are able to cover larger parts of your day/week/month and your mind simply keeps the important parts but lets the rest fade.

When you're younger, those models are less complete and larger parts of your waking moments are needed to build the foundations of these models, so you feel like time is slower since so much more of your time is kept "fresh".

I'm probably butchering his take on it, but I blame my own mental models for compressing away the finer details!