You wonder about how time flies during retirement. While not yet retired myself I can relate the experiences of many people who are retired. For example, to my father, and my father-in-law, the days just fly by. They both say that “they’re busier now than when they were working.” Which is, in truth, inaccurate. I believe they feel busy because the day passes so quickly for them.
(Short anecdote: My mother died a few months ago. My dad just recently handed me a stack of magazines that my mom had received as a subscription gift from me. The magazines were still in their plastic covers. I asked him why he hadn’t read them. He said he didn’t have time. Huh? My dad does absolutely nothing all day long. I cannot figure out why he doesn’t go bonkers with boredom.)
If I understand you correctly, your hypothesis is that the more routine and sameness is in one’s life, the faster time seems to progress. The sameness of retirement seems to support it, but in order for the hypothesis to be true, childhood must be filled with new wonders every day (which you state as part of your evidence).
I’d argue that childhood is just as routine as adulthood. Prior to school there is a very limited set of activities and people to interact with. Every day is pretty much the same. Then when old enough to enter school, the next eight to twelve years are also pretty much the same. Different class, same routine. One could actually argue that as an adult there is actually many more new experiences than as a child.
I lean heavily to the idea that the speed of the passage of time is directly related to the fact that as we age, each unit of time is a smaller and smaller percentage of our lifetime.