Most glaciers are over thousands of years old
The oldest glaciers in the world are located in Antarctica and are estimated to be about a million years old. Most of the ones we see in inhabitable places are probably younger, but being “young” for these giant chunks of ice is still thousands of years old.
Glaciers are large accumulations of crystalline ice, snow, rock, sediment, and liquid water. They are formed when the snow gets tightly packed together for an extended period of time. This causes it to become denser until it eventually becomes impermeable.