Chaim Yudkowsky point out in In Response to the Proposal Encouraging Our Young Men to Marry Very Young, it's not all positive.
He also makes the point that at that age, they still have some growing to do in terms of finding themselves. Now, that does vary, as some 19 year-olds are more mature than some 22 year-olds, but still the one-age-fits-all criterion that pushes for a uniformly young age as the ideal one for shidduchim disallows for individual differences. Another thing that does is the shidduch system in general. While he critiques it as being a product of the 1980s rather than of the idealized Litvish world of pre-war Europe, it really has grown far worse. In the 80s and even in the 90s, no one was sending out shidduch resumes - something that people now consider essential to the system. I've discussed this at length in
Also see http://kallahmagazine.blogspot.com/2011/11/you-dont-marry-shidduch-resume.html
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