In the US, 18 for rifles and shotguns, 21 for handguns.
Hand gun - 21; long gun - 18
It depends on your meaning of "have". To purchase a paintball gun the legal age is 17. Though if you have an adult present, you may purchase one at any age. So in theory, a 2 year old could have a gun.
21
To purchase a gun, no. Anyone can purchase a gun as long as they meet the age requirement, aren't a felon, and don't have any disqualifying misdemeanors.
In the US, legally you have to be 18, illegally there is no age limit.
18 for long gun, 21 for hand gun
There is no restriction on age for a sight in the USA.
Contact your local police for current information.
Assuming you are of legal age to purchase, black powder or smokeless powder for reloading can be purchased from gun shops. Yes, proof of age is required.
Since you posted the question on the internet, do an internet search on Maryland gun laws.
18 or 21 depending on if you are buying long gun or hand gun ammunition
You need to consult with a lawyer for a legal/current/correct answer.