A. LPR at Time of Filing and Naturalization
In general, an applicant for naturalization must be at least 18 years old and must establish that he or she has been lawfully admitted to the United States for permanent residence at the time of filing the naturalization application.  An applicant is not lawfully admitted for permanent residence in accordance with all applicable provisions of the INA if his or her LPR status was obtained by mistake or fraud, or if the admission was otherwise not in compliance with the law. 
In determining an applicant’s eligibility for naturalization, USCIS must determine whether the LPR status was lawfully obtained, not just whether the applicant is in possession of a Permanent Resident Card (PRC). If the status was not lawfully obtained for any reason, the applicant is not lawfully admitted for permanent residence in accordance with all applicable provisions of the INA, and is ineligible for naturalization even though the applicant possesses a PRC.
An applicant must also reside continuously in the U.S. for at least five years as an LPR at the time of filing,  though the applicant may file his or her application up to 90 days before reaching the five-year continuous residence period. 
B. Conditional Residence in the General Requirements (INA 316 (http://www.uscis.gov/ilink/docView/SLB/HTML/SLB/0-0-0-1/0-0-0-29/0-0-0-9898.html))
A conditional permanent resident (CPR) filing for naturalization under the general provision on the basis of his or her permanent resident status for five years  must have met all of the applicable requirements of the conditional residence provisions.  CPRs are not eligible for naturalization unless the conditions on their resident status have been removed because such CPRs have not been lawfully admitted for permanent residence in accordance with all applicable provisions of the INA.  Unless USCIS approves the applicant’s Petition to Remove the Conditions of Residence (Form I-751 (http://www.uscis.gov/i-751)), the applicant remains ineligible for naturalization. 
1. Nationals of the United States
The law provides an exception to the LPR requirement for naturalization for non-citizen nationals of the United States. Currently, persons who are born in American Samoa or Swains Island, which are outlying possessions of the United States, are considered nationals of the United States. 
A non-citizen national of the United States may be naturalized without establishing lawful admission for permanent residence if he or she becomes a resident of any State  and complies with all other applicable requirements of the naturalization laws. These nationals are not “aliens” as defined in the INA and do not possess a Permanent Resident Card (PRC). 
2. Certain Members of the U.S. Armed Forces
Certain members of the U.S. armed forces with service under specified conditions are also exempt from the LPR requirement. 
D. Documentation and Evidence
USCIS issues a PRC to each person who has been lawfully admitted for permanent residence as evidence of his or her status. LPRs over 18 years of age are required to have their PRC in their possession as evidence of their status.  The PRC contains the date and the classification under which the person was accorded LPR status. The PRC alone, however, is insufficient to establish that the applicant has been lawfully admitted for permanent residence in accordance with all applicable provisions of the INA.