I'm a military service member and a us citizen. Can I marry a illegal immigrant that is still a minor and get her citizenship?

It is true that minors do not acquire unlawful presence (ULP) in the U.S. before their 18th birthday. However, your fiancee still was not admitted and inspected into the U.S. While she may not have ULP at the moment (and shouldn't risk being her 180 days after she turns 18 years old), in order to adjust her status she will need status.

Note that it's important that she not acquire ULP after her 18th birthday because at that point she will need a waiver to re-enter the U.S. This is your fiancee's benefit by being a minor: if she departs before she acquires ULP, she can avoid applying for a waiver at a U.S. consulate. While you may try to get married in the U.S. (if state law allows it), your fiancee will need to depart from the U.S. and seek a nonimmigrant or immigrant visa to enter the U.S.

Please contact my office if you would like to retain our firm for legal services.

When you say, "illegal," do you mean that she entered the U.S. without inspection (referred to as EWI)? Or did she lawfully enter the U.S. originally (that is, was she inspected and admitted into the country)? Despite your military status, these questions will determine whether she can be granted permanent residence in the U.S., or whether she'll have to depart and obtain a visa at a U.S. consulate.

Also, regarding marriage to minors: if the person is not emancipated or within the legal age to marry within your state, then you may not be able to marry the person until she is legally allowed to do so.