How old is a dollar bill when you can no longer use it?

Answer 1

There's no "expiration date" on US currency. Bills remain in use (and valid) until they become too worn, at which point they're removed from circulation and destroyed. The US Treasury says that for a "typical" dollar in more-or-less continuous circulation, this takes about a year and a half to two years, though bills that last much longer (because somebody put them in a piggy bank or under a mattress instead of immediately spending them and returning them to circulation) are very common also.

Answer 2

A dollar bill can remain in circulation for up to 9-12 years before it becomes unfit for use due to wear and tear. However, even if a bill becomes too worn, banks will typically exchange it for a new one as long as more than half of the bill is intact.