As you age, time flies faster. The same applies to when you are busy. Psychologists have long studied how the perception of time varies with circumstances, but economists have barely touched the subject. If the perception of time depends on one's age, the fix is easy: adapt the discount factor to the life cycle (beyond what the probability of death implies, that is a separate matter), and you are ready to study the savings behavior across generations, etc. But this can be more subtle than that.
David Aadland and Sherrill Shaffer point out that the implied discount factor may not necessarily be exogenous. If you choose to have a busy life, you also modify your discount factor. If you are rational, you must take this into account in your occupational choice. I have certainly noticed how time has been flying much faster in the past years, and writing this blog beyond my normal work duties has certainly contributed to being busier than usual. I have asked myself whether the time spent on this is worth it, and now that I realize I may also reduce my enjoyment of time, I need to question it even more. But beyond these egocentric ramblings of mine, Aadland and Shaffer show that these consideration could explain why people want to scale back as they age the time they spend working, or why they retire earlier that previous generations despite expecting to live longer. They also find that unless people anticipate the changes in time perception, optimal plans are not time-consistent.