At 16, I met him in high school.
At 17, I realised he was pretty cool.
At 18, we started going out ie officially girlfriend and boyfriend.
At 19, we succumbed to multiple difficulties - no money, living in different cities to attend college, and parental disapproval on both sides - and broke up.
At 20, I went on a trip to England with my mother and spent most of the 10 weeks missing him. When I got home, I called him.
At 21, we got married.
28 years and 5 kids later, we're still together. Our 3rd child - who is 21 - will get married next month. Many people have said he is too young. His bride-to-be is 20. We have similar views to those expressed by mrsM and taught them to our children.

Not at all sure if our story is typical, but that's how it was for us.