Marriage is a contract, so the broader question is whether a 40-year-old guy and a 9-year-old girl can freely enter into any kind of contract that affects her life in a major way. Most societies have laws that prescribe at which age a person is considered an adult, fully able to make rational decisions and responsible for his/her choices. This age may vary (and I don't know of any Objectivist consensus on this), but for example I don't think any rational legal system would recognize or enforce a contract making a 2-year-old responsible for paying off a credit card debt.

Knew you were headed there, just waiting.
Now you're bringing emotion, speculation, imagination and 'Oh, do it for the children' into the discussion. You have no idea of the child's life with her family, if there's sex involved, or if it's more like adoption. You can't know. So if you can provide the full set of details of the families, the two individuals, the culture and level of civilization they both come from and live in--then you maybe can obtain judgement from others.

In nature, a human female is biologically ready for sex and childbirth at puberty, and it's only been for 100 yrs or so, that our civilization has progressed to the point that such an event has become a negative compared to how many hundreds of thousands of yrs and countless generations of humans unable to progress beyond natures imperatives.

So a 40 years old guy and a 9 years old girl can freely decide to marry and that is not right or wrong, but its up to their voluntary agreement?

I didn't say it was right or wrong. The rightness/wrongness of marriage is determined by the two people involved based on value for value.

Yes, but how do you, according to objectivism, justify this as a wrong practice? What are the objective arguments against it?

Although many call the practice pedophiliac (and there may well be some justification to that), it strikes me that much of the reason is custom driven by tribalism, poverty, and political/financial joining of families.

It's only been since the 1900's that the practice changed here in the US. Very few girls weren't married by 16 and even younger in some areas and a great deal of arranged marriages were seen as common in 'higher' society.

I do not have a definition of a muslim marriage. Im referring to certain news like those:

https://wikiislam.net/wiki/Contempora...
http://www.dailymail.co.uk/news/artic...
http://www.rawa.org/temp/runews/2011/...

Im assuming, even if exaggerate, they are not completely lies.

Define muslim marriage.