> A provider offers a service is responsible to ensure it functions within the law.
This is what I like to call "engineer thinking" (and I say this as an engineer). By this I mean it's absolute.
The law doesn't work that way and when it does, it tends to be terrible unjust and illogical (eg mandatory minimum sentences, three strikes laws).
Here's how it actually works: because of the nature of your service users need to be able to give consent for legal reasons. It may be possible to provide such a service to under-18s but the legal compliance may be a significant amount of effort. Or it may be impossible.
Already we've run into a problem because what about emancipated minors? They can, by definition, give legal consent. You may choose to ignore this corner case because it's too hard. That may be fine or it might not. You may be accused of unfairly denying services in certain segments of the population. Depending on your service, you may even run into legal trouble.
Assuming you're a good actor, you make a "best effort" to comply with the age restriction. You may ask for certain personal information or even run a credit check. Certain things can only be done in person, perhaps just the first time, perhaps every time.
If it ever comes up that you provided services to minors and there were legal consequences (civil or criminal), a court would ultimately look at the balance of evidence to decide if your actions were sufficient given the consequences of noncompliance, the harm you could've done and so on (note: the standards are different criminally and civilly too).
So if you provide a hook up app and just ask "Are you over 18? Y/N" and have lots of profile pictures of suspiciously young people, lots of mentions of high school and so forth then it can be decided that you had a reckless disregard for your legal obligations and be fined or charged criminally. A court may also decide your actions were "reasonable" (this word comes up a lot). For example: you tried to verify them with a Facebook profile (with efforts to make sure it wasn't fake), you monitored photos and profiles for certain keywords and so on. That could satisfy your legal requirements even if it wasn't perfect.
It's the difference between a bank who sometimes unknowingly facilitates criminal financial transactions vs a bank that has flagrant disregard for the law. There are banks that will charge you 15-20% for a wire transfer, for example. No legitimate customer would ever pay that. It's clearly aimed at people who don't want any questions or scrutiny. This will land you in jail. The first won't.