1. It would be highly illegal for anyone to sell a gun to a 16 year old. CNN regularly lies about guns and gun laws because they're pushing their left wing (ban guns) agenda and the truth doesn't support their agenda. Buying a gun at a gun show requires a background check if the seller is a licensed firearm retailer with a federal firearms license (FFL). If the seller is a private seller then federal law doesn't necessarily require a background check but it does say its a felony to sell a gun to anyone if you have reason to suspect that they aren't legally allowed. So it would be a felony to sell to someone if you had any doubt about them being old enough.
So the bottom line for your story is that it would be highly illegal for both the buyer and the seller. So for a 16 year old to buy a gun would be similar to them buying drugs or other illegal items. If the character is a good wholesome church boy who's never done anything wrong he'd have a hard time finding someone to sell him a gun. If the character is a criminal thug who's been in a gang since he was 12 years old, he would probably have no problem finding someone to sell him a gun through his criminal connections. Either way it would be a felony for both the buyer and seller, and the seller would probably demand 3x to 4x more money - so for example a basic used Glock 9mm pistol in fair condition might sell for $500 at a regular gun shop, but in this transaction the 16 year old criminal would have to pay about $2000 or more if you want the story to be realistic.
2. Buying ammo from a store requires age verification: 18 for rifle/shotgun ammo and 21 for handgun ammo. Some stores won't check ID if someone is obviously old enough, but for a 16 year old they would ask for ID. It might be slightly easier to obtain ammo on the street than the gun itself, but it would still be difficult and probably require criminal connections. So the most realistic story line is probably for your character to obtain ammo from the same criminal dealer who illegally sells him the gun.
3. Iron bullets would not work in most modern rifles and handguns. The closest option would be steel shot in a shotgun. This is readily available at most gun stores. Its a fair bit more expensive than standard lead shot, but its available. (Steel shot might cost $25 to $50 for one box of 25, compared with lead ammo which might cost $7-$15 for the same box of 25).
Legally when someone goes hunting and fires a shotgun over water they must use non-lead ammo for environmental reasons. So people who hunt ducks, geese, etc are required to use that expensive steel ammo, which is why most gun stores will have it available.
But remember the 16 year old character will need to use his criminal connections to get the steel shot, he can't just walk into the store and buy it.
4. No, you generally can't just make bullets out of random metal. The metal must be soft so that it forms into the grooves inside the barrel. That's why most bullets use lead or copper. There are some creative ways to get other metals into the bullet - for example an iron tip or iron core surrounded by lead or copper, or perhaps as various types of shot in a shotgun, but those are highly custom. You'd need to do a lot more research about firearm ammo before you could write intelligently about these items.
The bottom line is that if your story calls for your character to shoot something with iron, then your most realistic scenario is for them to purchase a 12 gauge shotgun and use steel shot cartridges which are primarily intended for waterfowl hunting.
For future reference it would be in your best interest to stick to stories about topics you are familiar with, and ignore everything you see in movies or on CNN regarding guns.