Here's why:

* Light going backwards: As you get older, your past (which is behind you) seems to move away from you, becoming "lighter" in the sense of less impactful and further away.

* Heavy forward: As you get older, the future (which is ahead of you) seems to weigh heavier, filled with responsibilities, potential challenges, and the realization of your remaining time.

Let me know if you'd like to try another riddle! 😊