At some point farthings ceased to be valid currency—after a certain date the government would no longer honor them as money. I saw a cartoon of a Scotsman with a huge bag over his shoulder, obviously filled with farthings, rushing to cash them in before the deadline.
A farthing was a fourth of a penny, discontinued in 1956. I am sure you could still redeem them for a while after that, but they might not be accepted as legal tender by retailers.