Mynet is reporting on an attempt by Rabbi Asher Eidan (Who?) - Supposedly a Mashgiach in some Yeshivot in Bnei Brak to make marriage legal at 15. His reasoning?
"There are many Hassiduiot that today break the law and marry their kids at 15-16. Parents to families with many children that have a difficulty to support their children financially, could marry their daughter off earlier, and she would be supported by the husband's family."
"יש חסידויות שכבר היום עוברות על החוק ומתחתנים בהן בגיל 15־16. הורים למשפחות ברוכות ילדים שיש להם קושי להחזיק את הילדים מבחינה כלכלית, יוכלו לחתן את הבת יותר מוקדם ושזו תעבור לבית בעלה".
Hmmm. And how is this 15-18 year old yeshivah bachur going to support his wife? how is his father going to support them, since I can only guess he isn't working either. Also wouldn't your son also marry some poor little girl, and ask you to support her?
Ok, lets keep reading the article. Rabbi Asher adds a few more reasons - "those girls will not be tempted to have forbidden relationships despite the fact that they could be married!". Well I'm fairly sure he has me on that point. The Talmud קדושין כט עמ' ב() does have Rav Hisdah say that he was better then the rest of his generation, because he married at 16, and had he married at 14 he could have told the Yetzer Harah to poke out its own eyes.
אמר רב חסדא: האי דעדיפנא מחבראי, דנסיבנא בשיתסר [שש עשרה]; ואי הוה נסיבנא בארביסר
הוה אמינא לשטן "גירא בעיניך
So I guess that argument has some strong roots. Certainly you can find enough historical sources showing Jewish women getting married at a young age, but I would hazard that in today's world that is simply not applicable. Firstly from a biological standpoint the advances in medicine have made child bearing age much much longer - as well as increasing the odds of those children to remain alive. The "need" to bear children young is simply no longer there. Secondly on the society level the amount of education one needs to succeed in the modern world is far greater then it has ever been before. I'll admit that neither one of these arguments really answers the "Yetzer Harah" argument, but I suspect that in a cost-benefit assesment they will have to do. Additionally I'm going to add one more controversial argument. Kids nowadays are just not expected to support themselves. They are not mature enough to take care of themselves (We don't allow them to drink, drive or have a gun), how can we expect them to take care of babies? Perhaps in a society where a 14 year old is working and has left the cradle of childhood, the cost - benefit argument against the Yetzer Harah will lean more towards marriage. Regardless Rav Hisdah's view is not the commonly held Halachic view.
We get a much better hint of what this is all about, towards the end of the article. It seems that part of the reason is to stop Haredi girls from trying to get an education before they get married. My own guess is that nothing will stop a girl from having any chance of an education - and perhaps asking uncomfortable questions, quite as much as having to stay home and take care of a screaming baby.