Dev,
I'll give you the same advice I'm giving to my daughter and to the young man she dearly loves, but can't date because of the age difference.
Remain friends. Friendship is one of the most important relationships in life and unless there is a serious betrayal, remain friends.
Then...you're 19, you're single. You have a the last vestiges of childhood to grow out of into full adulthood. Concentrate on that. Finish your schooling, learn your trade, develop your talents that you are going to need to be a fully competent, contributing adult capable of supporting a family. Go your way.
If, along your way, you meet a young woman that you find a "connection" with, by all means ask her out. See where the relationship takes you. Naturally, conduct yourself with honor and integrity, keep chaste, but see if the young woman is someone that might possibly become much more in your life. If she's not, then she's not. Allow the relationship to move apart, just stay honest with her, don't treat her badly.
Also, go out with other friends. Have a social life. You're young and independent, this is a great time in life to do things that you might not be able to do later.
When all is said and done, and your 15 year old best friend turns 18...that's less than 3 years, since she's already 15...and the two of you still have the attraction, then by all means, ask her out then. If your friendship remains true throughout your moving into manhood and her finishing high school, there is probably a very good chance you have a life-long mate in her.
My daughter's young man is now dating another girl. He made the decision to do so last September, and the relationship seems to be going well. He talks with me about his girlfriend, and I don't get the impression that he's ready to pop the question to her anytime soon, but she's cute as a button and they certainly enjoy each other's company. I don't honestly know if he'll still be single when my daughter turns 18, but then again, he still has some pretty special feelings for her. I know it was a very hard decision for him, but one I fully support. And, one my daughter, even though it kills her to think of him dating, supports as well.
But, I'll wrap this up with this: Don't try to make your relationship with this 15 year old girl into anything more than friendship at this time. Over two years is too long and both of you will be making some jumps in your maturity level. You might find big changes in you when you turn 21...that's when things really hit my daughter's guy. She might rethink things a lot differently when she graduates high school...that's when my best friend's daughter broke up with the guy she thought she would marry just as soon as she turned 18.
If you push for something more than friendship and then things change...you'll lose the relationship. Stay as friends, live life, move forward...and when she's old enough to make an adult decision with a fellow adult...then go for it.
