The interesting thing is that the US has one of the oldest currency systems. due to the US policy of not invalidating old currency, an old 1920's silver dollar

The interesting thing is that the US has one of the oldest currency systems. due to the US policy of not invalidating old currency, an old 1920's silver dollar is still legal tender, of course at a market value of several hundred dollars you would be mad to spend it as legal tender. A counter example is the uk pound, when the uk moved to the decimal pound all the shilling pound notes became invalid. so the us dollar system is older than the current uk pound.