I find this is very common in yuri, where a character will take forever to realize that she likes someone romantically/sexually, though I wonder how common it is in real life. I think people tend to know very quickly if they're attracted to someone, even if it does take longer to realize you love them. I can't tell if this represents a common phenomenon or just an affectation of the genre.
I'm going to regret getting so personal here, but whatever, yolo. By no means am I trying to speak for everybody, but think of it this way. For the most part we're raised from a young age to think we'll grow up and end up with someone of the opposite sex. Heterosexual is the majority, afterall, it's plastered everywhere you look. A young girl wouldn't necessarily realize she is attracted to another girl if she hasn't realized she's a lesbian yet. It's not really something you can put a value on. I personally realized yeeeears later that I had a crush on a friend when I was 13ish. Again, it's different for everyone, but at middle school age, it's not like you can tell the difference between attraction and just really wanting to be friends with someone of the same gender when the thought has never crossed your mind before. It uh, can take time.
It's funny, thinking back on it, I realized I wasn't into guys romantically (had zero interest in relationships), but the thought of liking girls wasn't really on my mind either. Even with getting awkward around some.