Just to quote from the ATF page, and I provided a helpfull link at the bottom of this page, 21 is the magic number...
5400.15 (Part III), accompanied by the required fee,
with ATF in accordance with the instructions on the
(a) The Chief, Firearms and Explosives
Licensing Center, shall issue a license or permit if
(1) a properly executed application for the license or
permit is received, and (2) through further inquiry or
investigation, or otherwise, it is found that the
applicant is entitled to the license or permit. The
license or permit and one copy will be forwarded to
the applicant, except that in the case of a user-limited
permit, the original only shall be issued. Each
license or permit will bear a serial number and this
number may be assigned to the licensee or
permittee to whom issued for as long as he
maintains continuity of renewal in the same region.
(b) The Chief, Firearms and Explosives
Licensing Center, shall approve a properly executed
application for a license or permit, if:
(1) The applicant is 21 years of age or over;
(2) The applicant (including, in the case of a
corporation, partnership, or association, any
individual possessing, directly or indirectly, the
power to direct or cause the direction of the
management and policies of the corporation,
partnership, or association) is not a person to whom
distribution of explosive materials is prohibited under
the Act;
https://www.atf.gov/explarson/fedexplolaw/index.htm
