6 months
You must always enter the US with a passport, or a valid immigration card.
Yes. As long as the passport is still valid.
Yes you can, as long as your passport is valid
So long as you had a valid passport, absolutely.
Yes, as long as you do not arrive on an airplane. You can use a U.S. Passport Card to enter Canada on foot, by car, train, boat, etc.
Even Infants can enter the US with their US passport. So there is really no age limit to one who wants to enter as long as they have appropriate documents.
Yes, any visit, no matter how short or long is required to have a Passport to enter Canada, same goes for the US
You can enter your country of citizenship as long as the passport is valid- even if it has 1 day validity remaining. However, as soon as a passport expires, you cannot board the aircraft using that passport.
Passports for minors are valid for 5 years.
No, as long as you stay under a certin period of time.
7 years
minimum of 6 months on a valid passport, of course you need a visa as well - check with the airlines as visa requirements differ from country to country