There is no minimum age for licensees. The youngest licensee that I've been able to find record of was five; his license was earned back in the days when we still had the Novice exam. With the end of the Novice license, all new licensees must now pass the (somewhat harder) Technician test, which would be somewhat difficult for most five year olds, and the youngest Technician I've ever heard of was nine. (Unfortunately, I can neither find, nor remember, any details about either of these two child prodigies.)
Nor are licensees required to be either citizens or residents of the United States. Resident aliens are just as entitled to have a US-issued amateur license as citizens are, and even nonresident aliens can obtain one if they have an address in the United States at which they can receive mail. (A PO Box or mail drop, or the address of a friend or relative who is willing to forward your mail to you, is sufficient for this purpose.) However, if you do not have a Social Security Number, you will need to obtain a Federal Registration Number (FRN), via the FCC's website, prior to taking the license examinations, or the VE team will not be able to process your application. It's entirely possible to obtain an FCC license without ever setting foot in the United States, although I'm not clear on why one would want to do this.
The FCC does reserve the right to deny a license or license renewal, or to cancel a license, if the applicant or licensee "lacks the requisite character qualifications to be and remain a Commission licensee". This is pretty rare, though, and requires pretty signficant misconduct.
As a side note, there is no prohibition on federal government employees being amateur radio licensees. While no federal agency may obtain an FCC license (federal agencies are required to coordinate their radio activities through the NTIA instead of the FCC), in general nothing prohibits a federal employee from being an amateur radio operator on their own time. Federal employees who may have a reason to use amateur radio frequencies in the course of their duties must be specially authorized to do so by the FCC or by other relevant authority.
This post has been brought to you by pool questions T1D02 and T1D03. Section references above are to Title 47 of the Code of Federal Regulations, browsable via the GPO Access eCFR service.