Would caps and balls be considered ammunition under Massachusetts law? They’re separate items I think.
It would appear that they are considered "black powder" ammunition, and that it is not regulated as regular ammunition. It's clearly legal for a 20-year-old to possess it, but purchasing it is less clear:
MGL Chapter 140
Section 129C.
No person, other than a licensed dealer or one who has been issued a license to carry a pistol or revolver or an exempt person as hereinafter described, shall own or possess any firearm, rifle, shotgun or ammunition unless he has been issued a firearm identification card by the licensing authority pursuant to the provisions of section one hundred and twenty-nine B.
No person shall sell, give away, loan or otherwise transfer a rifle or shotgun or ammunition other than (a) by operation of law, or (b) to an exempt person as hereinafter described, or (c) to a licensed dealer, or (d) to a person who displays his firearm identification card, or license to carry a pistol or revolver.
The provisions of this section shall not apply to the following exempted persons and uses:
(p) Carrying or possession by residents or nonresidents of so-called black powder rifles, shotguns, and ammunition therefor as described in such paragraphs (A) and (B) of the third paragraph of section 121, and the carrying or possession of conventional rifles, shotguns, and ammunition therefor by nonresidents who meet the requirements for such carrying or possession in the state in which they reside.
Of course, the usual caveats about reading MA laws as though they mean what they say is always required.
GOAL's take on the matter:
Even though a person does not need any license to purchase or possess a primitive arm, one must show proof of majority (over 18) for purchase or possession. There are no reporting requirements for antique firearms either at the federal level (Form 4473) or at the state level (F/A-10). Because of conflicts in state law, however, a Massachusetts dealer will ask for a license or card in order for you to purchase ammunition for the primitive arm.
www.goal.org