Should I Ask Him?

I am 24 and I have been living with my boyfriend for almost two years. Everything is perfect and I`ve never had had such teamwork with anyone else in my life. He has brought up the idea of marrige to me a few times in the last year but it was nothing serious. I`m thinking of bringing it up again and I would like to get married. I know he wants to marry me but I am afraid to ask because he is only 21. As far as maturity and responsability goes he is old for his age. I`m just wondering if I should go for it. My heart says yes, yes, yes but I am afraid. What do you think?