Correct spelling, explanation: fourteen is the correct spelling because it comes from the number four and the suffix -teen. 14 is always spelled with u, in contrast to 40 (forty), which – for unknown reasons – lost the u in spelling around the 18th century. Number 14, however, has never lost the u from four, so fourteen is the only acceptable spelling.

Definition of fourteen:
numeral, cardinal number between 13 and 15
Jenny was a prodigy; she graduated high school at the age of fourteen.
After working fourteen hours straight, John was exhausted and ready for a good night’s sleep.

Incorrect spelling, explanation: despite 40 being spelled without u (forty), the number 14 is still spelled with this letter: fourteen. We do not know exactly why forty is no longer written as fourty, while fourteen is simply the number four with a -teen suffix. Therefore fourteen is the correct spelling and forteen is incorrect.

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling