The riddles of the ancients

Riddle 1

This thing all things devours:
Birds, beasts, trees, flowers;
Gnaws iron, bites steel;
Grinds hard stones to meal;
Slays king, ruins town,
And beats high mountain down.

Riddle 2

What comes once in a minute, twice in a moment, but never in a thousand years?

Riddle 3

 I’m tall when I’m young, and I’m short when I’m old. What am I?