21, same as for any FFL, you have to be able to answer the "Applicant is at least 21 years old"-type question correctly or you will be denied.
My application was signed and in mail on my birthday, had the license 6 weeks later (the ATF told me that it was delayed 2 weeks because MD drags their feet when doing background checks for the ATF, most other people get theirs in 4 weeks at this time).
Kharn