Until 35 for "professional induction" (my term)...
Sigh..
But of course if there was a desperate shortage then they'd take anyone.
http://www.sss.gov/PDFs/MSSA-2003.pdfThe Military Selective Service act
As Amended Through
July 9, 2003
...
Except as otherwise provided in this title (sections 451 to 471a of this Appendix), every person required to register pursuant to section 3 of this title (section 453 of this Appendix) who is between the ages of eighteen years and six months and twenty-six years, at the time fixed for his registration, or who attains the age of eighteen years and six months after having been required to register pursuant to section 3 of this title (section 453 of this Appendix),
...
Criteria]
Except as otherwise provided in this subsection the President is authorized, under such rules and regulations as he may prescribe, to provide for the deferment from training and service in the Armed Forces of any or all categories of persons whose employment in industry, agriculture, or other occupations or employment, or whose continued service in an Office (other than an Office described in subsection (f)) under the United States or any State, territory, or possession, or the District of Columbia, or whose activity in study, research, or medical, dental, veterinary, optometric, osteopathic, scientific, pharmaceutical, chiropractic, chiropodial, or other endeavors is found to be necessary to the maintenance of the national health, safety, or interest: Provided, That no person within any such category shall be deferred except upon the basis of his individual status: Provided further, That
persons who are or may be deferred under the provisions of this section shall remain liable for training and service in the Armed Forces under the provisions of section 4(a) of this Act (section 454(a) of this Appendix) until the thirty-fifth anniversary of the date of their birth.