I Was In Love at 15
I'm not sure exactly how to answer this question without sounding like every other older person that you know, that sounds like they could not possibly understand you or what you are feeling. So I'll probably do exactly that, with the hopes of also letting you know that I was once 15 years old and very much in love with a boy.
I Believed It Was Forever
I am now almost 29 years old (next month) and I have the ability that only a few added years can give me as to the years that I've already lived and you have in front of you. At age fifteen, I loved with all my heart.
My sweetheart was truly the love of my life, and to this day I have never forgotten him nor will I ever. He was my first love, and I believed it was my forever love. For me, that wasn't true.
However, I will not give advice to you and tell you that you are too young to get married, that you have a lot of life ahead of you because you probably already truly know this in your heart or you wouldn't be questioning it.
You Will Change
What I will tell you is that the next ten years ahead of you are going to change who you are. They are going to change how you look at life, at other people and most of all at yourself. Whatever experiences you acquire along the way will combine to make up the person that you work towards becoming. If getting engaged at age 15 is something that you feel you are ready for, then that may be an experience that helps to mold the person you become.
Your Marriage Won't Last
Since you did ask for advice, I will tell you. I don't believe that your marriage would last if you get engaged or married this young. The ONLY reason that I say this is not because I don't believe you love each other. It's more that you have yet to define who you are and what you want out of life yet. Right now, you want to define those things by being engaged. In two years, what defines you might be entirely different.
I'm a Different Person
When I was 15 and people told me how young I was, I never believed it or understood it. And now that I'm 29, I know it's true. I'm not 15, but I've watched myself change enough since I was 15 to know how much I have yet to experience and change in the next ten, twenty, thirty years. I also know that the person I am today is very, very different from the person I was ten years ago.
Love Yourself Enough To Grow
You love your boyfriend. Hopefully you also love yourself enough to give BOTH of you a chance to grow and learn first. You may very well grow together, but I do believe you will have a better shot at making your relationship work if you allow yourselves to be young now, when you are.
You have a lot of time to be an adult. Be 15. You only get to be 15 once.