Uncomfortable

There's a friend of mine who I've known for quite a while.

We used to be close, not really anymore; but that's not the point.

I've known his sister since she was 10, and now she's like 17 or something.

It's really weird for me when I hear about people that I know and are at my age take interest in her.

I guess it's not inappropriate or anything now, but I still have this impression of her as a little girl and it totally throws me off.