How do you tell the age of a dollar bill?

The closest you can come to telling the actual age of any US bill is by looking at the Series date on the front. This isn't the actual year the bill was printed, but rather what year the full plate was approved. A series year typically changes only when a major design change is implemented, such as the recent redesigns of the $5-$100 bills, or when the Treasurer's name changes; a new Secretary of the Treasury only rates a letter appended to the series year. Thus a Series 1999 dollar bill may have been printed in 2001.