If a 6-year old child is brought into the US from Mexico illegally how can he work legally and become a citizen when he is 18?

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when they are 18 they need to take a civil servise test about American history and other things, if they pass they are leggally a us citizen.  

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Actually, I believe the above answer is incorrect... if you're here after an illegal entry (brought here as a child) then you are in the same boat as everyone else here with an illegal entry. You have to return to your home country to adjust your status, and if you just walk into ICE (formerlly INS) to attempt to take a test, get a visa, etc, you will first be deported, THEN in your home country you can apply to come back... AFTER a three year waiting period for having illegally entered the first time.  

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I agree with the second poster. I do not think that they will give you a "green card" just for taking the civil service test, (or, is that something I don't know about?). The best bet for you (and any other illegal immigrant) is to get a visa or "green card" through a relative, more precisely, a parent or sibling (over 21) who has a "green card" or is a US citizen.  

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I would add that the Civil Service Exam is actually an employment test to qualify for certain types of Federal government jobs. It's totally different from the test on Civics and American History required of legal residents to gain citizenship.
Yes, that first answer is totally incorrect. Once they are 18, they are supposed to return to their country of origin, regardless of the fact that they've been here for many years and possible possess a high school diploma and have been good (illegal)citizens. I have several friends that are in this same situation. They were brougth to the US by parents. They went to school here, learned English, graduated from high school, and now have no future ahead of them here. Culturally, the are American. Some of them have no connection to the countries and towns they came from. With this push to send back all the "illegals", I ask you, what about this special population? Why are we wasting tax dollars that were spent on their education by sending them back? We should harness this untapped resource.
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There is some provision for this in USCIS ie. Federal Laws on Immigration and Migration. It may be possible to gain legal citizenship due to the number of years some one is resident. It is all very complicated and the rules have changed constantly on this-and will change again. But basically, there is a provision that a minor whom is illegal can gain domesticity under the law. It would take me many hours to type out all the provisions and proofs required, for it is a very long bureaucaratic process to complete. You need to consult a immigration lawyer on this one, it is complicated!
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ATTENTION: NO. YOU NEED TO DISCUSS YOUR OPTIONS WITH A LAWYER. TO MY KNOWLEDGE THERE IS NO WAY TO GET ANY STATUS. YOU WILL BE DEPORTED. IF YOU ARE IN THE HIGH SCHOOL LOOK FOR A "DREAM ACT" SUPPORT GROUP AND GET THE RIGHT INFORMATION.