What are current circulating coins made from?
Quarters and dimes are composed of cupro-nickel clad, with a pure copper core, and an outer layer of a 75 percent copper, 25 percent nickel alloy.

Nickels are made from the same 75-25 alloy, and the cent, once a copper coin, is now composed of copper plated zinc. These cents are less expensive for the Mint to manufacture, and at 2.5 grams each, weigh about 20% less than the cent previously minted of 95 percent copper and 5 percent zinc, which weighed 3.11 grams.

What is the life span of a coin?
The approximate life span of a coin is 25 years.

What happens to United States coins that are no longer fit for circulation?
Those coins are classified as “uncurrent” or mutilated. Mutilated coins are coins that are chipped, fused, and not machine countable. Mutilated coins are only redeemable through the United States Mint.

Uncurrent coins are coins that are worn yet recognizable as to genuineness and denomination, and are machine countable. Uncurrent coins are redeemed by the Federal Reserve Banks, then forwarded to the Mint for disposition.

All uncurrent or mutilated coins received by the Mint are melted, and the metal is shipped to a fabricator to be used in the manufacture of coinage strips.

To read more facts regarding US coins, check out the following link:
http://www.usmint.gov/faqs/circulating_coins/