I'm 37 years old. I lived with my wife for a year before we were married. Been 10 years, still going strong.
I lived with one previous girlfriend for 3 years before that, and another for 5 years before that. It's always been my decision to do so, and in this day and age (in America), I cannot believe anyone would make a decision of that magnitude and potentially devastating consequences if you don't even know if you can cohabitate successfully. What happens after you're already married, maybe have a kid or two, and then find out 2 or 3 years later that you'd be happier on death row having hacked the bitch into little pieces?
Just as our parents and grandparents retired from the very first company they were ever hired at, they stayed married through thick and thin no matter what.
But now, we change jobs every 6 months and nobody blinks an eye, and we treat marriage the same way. It's not sacred anymore. The divorce rate in the US is some 50%. It's for that reason that I wanted to be as confident as I could be that my marriage would work.