The passage of time seems to accelerate as we age. Why is that?

This is a consequence of changes in the brain due to aging. It is, in effect, a kind of shrinking of capability. In effect, having a sense of time lasting longer creates a bigger platform to experience things and have more productive thought and awareness and formation of memories about what is taking place because there is simply more time to do so, one can focus better, absorb more, and have the luxury of paying greater attention to detail before being distracted by unfolding events. So, in effect, it is the enhanced capability of the young compared to older people.