In theory, US banknotes remain "valid" forever.
At one time, it was illegal to own US gold certificates, but it's now legal.
http://en.wikipedia.org/wiki/Gold_certificate
Gold and silver certificates are no longer redeemable for gold or silver, but can be converted into equivalent "paper" notes. (They usually have a higher value as a collectible note.)
The US government doesn't like physical currency and would like to require everything to be done with checks and electronic transactions so they can be sure they can collect taxes and otherwise control the people's money.
The US government has stopped printing all denominations of more than $100, but will still exchange them for current denominations, and destroys them if they pass through a US bank.
I would not be surprised if the US government doesn't decide at some time to "repudiate" some old notes, but allow you to convert them to the current notes. They would claim it's to fight terrorism, drug smuggling, and counterfeiting. I'd expect them to require you to fill out some paperwork to exchange large amounts of old currency for new.
One could speculate that the real reason is not to fight terrorism and drugs, but to collect taxes, and to keep control of the money supply to help keep the peasants in line.
There's been a lot of talk about the US government wanting to get rid of the $100 bill to cut down on the "underground economy," but this has been resisted by a lot of people.
It's been said that US $100 bills are the backbone of the underground economy world wide. In once sense, the US government would like to get rid of all the $100 (and maybe $50) bills to make sure they have better control of the money worldwide. One problem with this is that there are a lot of $100 bills circulating in foreign countries for their underground/cash economies. If people overseas switched from, for instance, US$ to Euros, this could have a bad economic effect. The US government would have to buy a lot of Euros to replace the US bills being used in the worldwide underground economy.
The EU produces a 500 Euro note. There has been some debate that this is so large in order to allow the EU to transition the underground economy to Euros and get some of the "free loan" benefit of having your currency being used out of the country.
I believe EU notes DO expire and become worthless some time after they are replaced by a new issue unless you exchange them before the expiration date.