You may be any age provided you are with a parent or legal guardian.
You must be 18 to purchase and own rifles and long arms.
You must be 21 to purchase and own handguns.
18 years old.
Depends on how old the shotgun is.
http://www.wikihow.com/Shoot-a-Shotgun
Depends on your parents
Get your brother in law to shoot it for you.
Yes if they have a liscense or a legally liscensed parent or gaurdian.
A shotgun can shoot up to 80 yards it it depends on what the choke is on the gun.
a 12 gauge shotgun can shoot about 30 yards accuracy
You should be OK with standard loads.
No
Depends on how old it is. Have it checked over by a good gunsmith
Pellets or slugs. shotguns usually shoot shot or pellets, but you can shoot slugs. in Indiana we use a shotgun to hunt deer and use shotgun slugs, better to use a barrel with no choke in it. cylinder bore preferred.