18 is used because it's really the minimum "safe" age. Even states within the US all have different ages of consent, and travelers are still tied to the ones that their state applies to them (even outside of the US). It's a personal preference for me too; the AoC may be 16 in my state and 16 wherever you live, but since there's no way to be sure of where anyone I only know on the internet lives, I'll stand by 18 to keep myself and anyone I RP with out of trouble.
