No specific moment; a series of choices really.
We first got together when I was 14, he was 17. We've been together 11 years, and have grown up together really. I always suspected we were a pretty good match - so much so that at 16 I freaked out that I didn't want to have already met the person I'd spend my life with and broke up with him! Thankfully changed my mind on that one, and we're extremely happy together. I can't imagine us ever splitting up, but then you never know the future.