There are legal and, for people who care, religious differences.
There's also some reasons to believe that cohabitating before marriage (to the same person) leads to higher rates of divorce: http://www.nytimes.com/2012/04/15/opinion/sunday/the-downsid...
What's different about it? I'm not 100% sure. But none of the guys I lived with (for years) caused me the kind of angst that my husband did at first, when I moved in with him and immediately got married a few weeks after. (Prior, we didn't really live together before marriage except for weeks/months at a time at the others' apartment, living in different countries.) I'd never had a terrible "adjustment period" before, but did this time… and believe me, on paper, previous boyfriends were way more annoying than my husband was, even in those first few months. We got through it, obviously. I'm grateful for him every day.
I'd always been a committed and faithful girlfriend to the others but it just isn't the same as being married. Which surprised me as much as anyone.
Intellectual types underrate the importance of ceremony. I now believe that's a mistake.
The problem with seeing this as a causal relationship from cohabitation to divorce is that there's no control for religious belief.
The same religious beliefs and social structures that deter pre-marriage cohabitation also create pressure not to divorce.
"...however, studies have shown that the effect is not entirely explained by individual characteristics like religion, education or politics. Research suggests that at least some of the risks may lie in cohabitation itself."
But I agree that getting married as young as possible is a good idea. (And I don't care if you call it marriage, or living together - it's effectively the same thing.)
And Scripture recommends getting married young BTW, so your distinction doesn't exist.
People who get married later in life have a hard time adjusting to having someone else in their life who needs to be their number one priority.