Here's why:

* Eats rocks and levels mountains: Time causes erosion, weathering mountains down over millennia.

* Rusts metal: Time leads to oxidation, which causes metal to rust.

* Pushes clouds across the sky: Time is associated with the movement of weather patterns and the wind that drives clouds.

* Can make a young man old: Time is the ultimate passage that marks the aging process.

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