How do you know it's gonna last?

Just wondering how you know it's gonna last. My boyfriend had been with a lot of people before me and says that I am the one and that he sees us living together and he wishes I was a little older so we could live together. ( I am 16 and he's 18 ) he's my first and I love him more then anything, if I ever lost him I would truly lose myself. How do you know if it's gonna last or not? 
Please no one saying " enjoy being young" " don't worry about it yet" or anything like that. I just wanna know how you know it's gonna last :)