I find pennies,nickels,dimes,quarters from all years. 2013,1974,1982,1990, 1967 etc. Dollars I found a few from 1999. What happens to them after so long do they recycle them? Destroy them? Anybody know? I found a $2.00 bill I kept. Like I have NEVER found like a 1982 1990 dollar bill.

The banks send back worn bills to the U.S. Treasury, if I'm not mistaken, where they are shredded. Because of the wear, they tend to not stay in circulation very long -- and generally, the lower the denomination, the more frequent use and potential wear.

If you use a lot of cash, sometimes you will encounter older bills. Around the year 2002, my bank gave me a $10 bill from 1951. It was in nice condition and didn't look out of place with the rest.

What happens to old coins? Do they get sent back to the Treasury to be melted down and recycled or do they remain in circulation indefinitely?

Paper bills are shredded and recycled. I assume that coins are remelted and made into new coins.

I've never heard of old coins being returned to the mint, although it's possible. My guess is that the U.S. Mint obtains the metals it needs to make new coins on the open market.

Except in cases of deliberate abuse of a coin, they last indefinitely. Although the lowly penny has been tossed away at times, as an example of why new coins are always needed for circulation.

Older silver coins are either in the hands of collectors & dealers or have been melted down by private individuals for their silver content. The change to clad coinage (beginning in '64) happened less than 50 years ago, so a lot of those coins have survived in decent condition as they're not terribly old -- and they don't have significant value to be melted. The change from copper to zinc alloy pennies happened in '82 IIRC, so there may be some melting down of older ones by individuals (pre-1959 wheat ears are collectible though).

If you check your change often, you will occasionally find a 1940 or 1941 nickel. Those years had a couple of large mintages.