When I was a child, there was “going steady”. You actually asked the girl if she would go steady with you and you became an official couple. Sometimes it meant she wore your school jacket.
When I was a teenager we were in the midst of the Haight-Ashbury phenomenon. Nothing was official. In high school you might have a boyfriend or girlfriend you were "serious with”, but it was never officially announced. It just happened.
When I was in college having someone you were serious about was something you never discussed. We were serious about ideas and our education and life. A girl might have a guy she loved and spent time with and she might even be living with him, but it was something on the side, not the center. People would think you were weird or needy if you were focused on defining that relationship.
After I married L., I didn’t pay much attention to what the latest form of establishing a serious romantic relationship was like, so I can’t fill in the ensuing decades. But I am intrigued by the latest one I encountered this week: a facebook notification that you’ve been sent a relationship request. A young friend recently posted his delight at having received one and changing his facebook status to "in a relationship with _________". Good heavens. Officialdom with the click of a button, miles away from the object of your delight and affection.
Things do change.