Well, you just have to say that all characters are 18+, even though, for example, consensual sex is legal at 16 years old in the UK (and even lower in some jurisdictions). Age of consent for sex, alcohol, tobacco, and even caffeine is always interesting, and hypocritical.
You can't consent as a sixteen year old and sign a contract to enter a financial agreement with an 18+ year old, but you can fuck the guy or gal holding the contract documents all day.
You could be 16 years old and fuck a room full of fifty guys who are all fifty years old, with dildos and such, every weekend, but you can't legally watch porn, buy a pack of cigarettes and an energy drink, or go into a sex shop, for the next couple of years.
One has to assume this game is really catering for an audience who "likes 'em young, but still legal-ish."