Who is misinforming them ?

Feeling a little defensive, aren't you? Assuming you didn't just make this up because you want people to reassure you that living at home is ok, the teenager clearly wasn't talking about years, but about maturity. Some people live with their parents for financial reasons, which is fine. But in some cases, it allows them to put off maturing. They don't take on financial responsibilities, aren't serious about building a career, and continue to act like teenagers. That doesn't happen just because they live at home, but living at home doesn't push them to grow up mentally. 

American culture looks down on this. It is not uncommon for families of 3 generations to live under the same roof. Latinos, Asians, Albanian etc. This is typically culturally driven. 

probably other kids in school are telling them that

Social media, where the US now gets more then half of it's news and information. Doesn't seem to matter that we all know it has just as much misinformation being thrown around.  It's insane, our mindset that our sources are honest, or our streams are honest. It's a freaking crock and it's dangerous.

"I just want to know who told teens this?"

Why are you generalizing about all teens?    If you want to know who told THIS teen, you'll need to ask THIS teen.

It's possible no one told the teen anything about this subject.   What the teen said is 100% true for many people including his/her own self.   The teen lives at home and is not an adult.   And this is probably true for almost all of this teen's peer group.

So before you go around knocking a pupil down for being wrong, pull your head out of your own butt and consider the life experience of the person you're talking to.

ETA:  "So if a man in his 30s lives with his parents he’s underage?"   Nowhere did I say that.  But in your case it's figuratively true.   Have a nice day.