I believe that the age observation that you make is the key. My wife and I lived together for a couple of years before we married. That started in the mid twenties for her and the late twenties for me. We married in her late twenties and my early thirties. Last January was the 27th anniversay of that event.
Should I still be looking for the other shoe to fall?
Well, I dunno. I told Mrs Wbill that I was planning to hang on to her until she was 80, so that I could trade her in on four 20s.
She said "Go for it!". I suspect that she figures (correctly) that I won't remember why I wanted the four 20s to begin with.... :-)