It's almost as though we need a "manslaughter" charge of the sex world when it comes to the 18+ club problem.
But on the other hand, I don't really support flexibility on this. You should be responsible in who you lie down with, anything less should be discouraged, by the law necessarily since people (especially the greedy and the stupid) don't necessarily lessen to anything lighter or less forceful than it.
Yes, you can't legally verify if a person is the legal age if it's near that point at times, but you need to have some wits about you.
Stupidity, such as that of not thinking about what you're doing when you're about to have sex, CAN be a crime, and so can being deceived by a clever underage person wanting sex (we should note just how common it is - most young girls these days lose their virginity at the appalling age of 13, even lower in some countries).
We should also note that, let's face it, sufficiently under-dressed and attractive women can get into nightclubs often without even showing ID, because bouncers are encouraged to do that by owners. It's part of the culture in many places. Some of these women are probably underage. So the 18+ club idea in principle isn't even certain enough to rule that "everyone having sex in an overage club must not have known that the person is underage". That's too exploitable a loophole, it would be used very wisely by real perverts.