18

18 is the only number that is twice the sum of its digits.

18 is the number of holes in a stipulated round of golf

18 is the number of chapters into which James Joyce's epic novel Ulysses is divided

18 is a slang term used in Soccer for the Penalty Area

18 is the number of wheels on the most common type of tractor-trailer truck, which are hence often called 18-wheelers

18 is the customary percentage to tip a waitress in a restaurant by groups of four or more

Australian rules football is played with teams of 18 players.

18deg. Celsius is usually considered a comfortable room temperature.

In the UK, the names of race horses can be no longer than 18 characters, including spaces.

On Earth, the length of a day 900 million years ago was only about 18 hours. - Science magazine

Don't you feel smarter now? Haha! If you haven't guessed, I'm 18!!