15 means having fully bloomed into sexual maturity, at the final fifth Tanner Stage. That's backed up by biology and facts. Any other age is just an arbitrary number, most likely based on prudish or compromised thought. So that image is in fact self-telling. 18 is in fact just a number (and not even applicable in the US, as US age of consent is 16-17 for most states). If you're talking mental maturity, that age would have to be 25-27. Good luck enforcing that Prohibition.