Wow first of all great question
Great Question BTW
Yes an 18 year old should be able to own a firearm in the US no matter if he is in foster care as long as he fills out the forms and has it registered and a carrying lisense and a backround check.
Here is the kick in the teeth you cannot buy a firearm if you are under twenty one (at least in indiana but im pretty sure that is federal) it must be given to you with the proper paper work.
EDIT
to carry a concealed weapon you must have a concealed weapon permit but if you truly felt like it if you had an assault rifle you could walk down the street with it loaded and the police might question you and ask for some identification to prove you are 18 but be warned they can arrest you if you are not holding it properly the safety is of or if your plain stupid and point it around im betting the will probably taze ya or worse.
