That's a misunderstanding, and unfortunately taken by some weak heart-ed Muslims as something good to do. Islam did not come up with that, people of all beliefs (and non beliefs) thru out history did this. It may be true that Muhammad had one of his wives a 9 year old (there are controversies actually), but that does not mean anything. Just because Muhammad did it, it does not mean it is something to follow. What we follow of Muhammad is the habits he lived with, and doing something once and never again is far from that. Marriage is a social practice, not just religious. It is also wrong to assume ill of the nature of that marriage, since that specific wife loved Muhammad so much that she reported his Hadeeth along with other things Islamic history talks about.
Also please keep in mind that this thread is about innovations found in the Quran and this point is not part of the Quran. Getting out of this requirement in further posts is considered going off-topic, and that's against the rules, unless it is proven it is mentioned in the Quran in the reply. Your are more than welcome to start this discussion in a separate thread so this one can take its course, and I'm sure Muslims of RF will respect that, as long as it is discussed in the right manner. We're here to share beliefs and socialize at the same time. Nothing is better than having that in a good an welcoming spirit. Bless you both.