My theory, which is logically consistent with yours, is that as we get older we spend more of our cognitive cycles thinking in terms of abstractions rather than using our senses to process direct experience. This is partly because as we get older we tend to immediately place things we experience into categories and then move onto processing these abstractions rather than continuously observing reality directly and unfiltered. So the other part of the theory would be that when we think in terms of abstractions, time goes by much faster than when we focus on direct sensory experience. It's likely that novelty also plays a role here as a mediating variable, in that we tend to pay more sustained attention to novel phenomena rather than immediately placing them into conceptual buckets and moving onto the next thing.
I also like this theory because it's consistent with how drugs that alter our sensory perceptions and make us focus more on our perceptions also tend to subjectively slow down time. Anyway I have no idea whether this is correct, but IMHO it's a much better theory than the original, which isn't especially logical and doesn't even really make sense.