I've dated a girl that was about ten years younger than me, plus a couple of years or so. Let's not get started in on that, but she was mentally a lot older than others gave her credit for.
No, I won't be repeating that with anyone under the age of 21, because I'm older now. No less wiser, but older. So you can rest assured this is more of a big brotherly type friend situation, and you're going to play the part of the little sisterly sort. And if anyone threatening comes through the door that we haven't invited over? We're going to bloody well kill them, together. And it will count.
In fact, if I've popped out? I want you to kill them without hesitation and with extreme prejudice.