What worries me more, is what if she isn't even 19. What if she is some lonely school kid who thinks this is what 19 year olds do?

At the end of the day Mac, it's down to you. If your happy just chatting to this person, whatever they might be then carry on.
But be aware, this may not be quite what it seems.
Personally, I wish you all the best and hope you get your happy ever after.