Look, one thing that you should learn is that once you have been friends with a girl for such a long time, forming a romantic relationship is very hard and such an attempt might end your friendship (I say this coming from personal experience). On the other hand, you two have been young for most of your friendship and perhaps she might have recently started looking at boys as potential partners (even though this is unlikely).

Anyway, let's cut straight to the point. Don't try to ask her out and accept she is your friend. Yes, you read it right. Reality is not like the movies or soap operas and you might seriously regret destroying such a great friendship for nothing more than a fantasy. If you have never been in a relationship, it is hard to understand that it takes more than just seeming compatible as friends to make it work. Hopefully, experience and time will show you this.

I am not going to give you detail advice on girls, but the minimum I will tell you is that there are hundreds of girls that you can be attracted to and have a happy relationship with. You don't have to fixate on this girl that you have invested so much time on to build this extremely close friendship that seems will last for many years and provide you with support when you need it. You will be an adult one day and you will go through tough times, and guess what, nobody is going to give a crap except your friends and family, and many of these will be friends that you make during your younger years. Would you like her to be there for you when you get fired from a job? When you are alone in your apartment and just wish you could go out have a coffee with someone? I bet you do. But she won't be if you try to pursue this fantasy of being in a relationship with her. I know in your mind it seems perfect, and it is extremely difficult to break out of this mindset. But please try, you might regret in the future, but I can guarantee you that you won't regret it as much as ending your friendship for pursuing a relationship that might not work (and like I said before, it takes more than just being compatible as friends).

On the brighter side, perhaps one day in the far future, she might consider you as a potential partner since she might be at a phase of looking for a more long term relationship. But take this with a grain of salt as this is just my intuition. Also, who knows, she might make the first move one day and then you can confess your feelings.

There is, however, one exception to what I have said. If you are willing to end this friendship with little to no regrets in exchange for the potential of experimenting being in a relationship with her, then go ahead and ask her out asap. In that case, the days are passing by and you want to get most out of your potential time together.