"I had to guess a bit as to which M7 is being referred to in the question. If the question is referring to the WWII tank the M7, then yes, the tank would be legal here in the US, but it would not be...
Monogamy is legal in all 50 states. Mono- meaning one implies that you would only be married to one person. Are you meaning to ask about polygamy?
Yes, pursuant to Roe v. Wade.
If you're talking about iphones, no, although apple tried to make it illegal.
It's 18 or 21, but it depends on local state law.