In Greek-based number-naming systems, 18 is associated with prefix "octadeka-", and with prefix "octodeci-" in Latin systems. In the Misalian seximal system, this number is called "thirsy", because it's three times six.
Since it is also the age of majority in most countries, it is associated with adult-only media and the age of consent in much of America. It is one of few non-single-digit numbers that has its own emoji: 🔞
- OEIS, sequence A002808 (accessed 2020-11-13)