"People on Reddit say that those that have the 18 marker have to be adults to get those accounts and the things she comments in posts in different subjects aren't minor-appropiate."
One. Being marked mature on Reddit can happen automatically. It happens when someone posts something NSFW (....something stated several times). One can have their account labeled mature and not even be aware of it.
Second. It's....not unusual for minors to engage in things that are above their age level. For instance, I'm a big Mortal Kombat fan, and have been since I was a young child, despite the Mortal Kombat series being known for...not being child friendly, we'll say.
And if you go through said Reddit account, this post can be found.
This was posted in October. And you were banned in July. And, as said, your account is automatically marked mature if you post something NSFW. Regardless on if only adults can access it is the least of our concerns here, X9.