NOTE: The details and information pertaining to Illinois ammo laws are meant as a reference point only. As you're likely aware, Illinois firearm laws change frequently so be sure to consult your local law enforcement agency or a lawyer before making any final firearms related decisions.
The State of Illinois has several ammo laws that shooters need to be aware of. First, like the federal government, the state prohibits armor-piercing ammunition. Next, Illinois requires residents to obtain a Firearm Owner's Identification (FOID) card in order to buy ammo legally. Even if you are buying ammo outside the state, you will need a FOID card and a driver's license or state-issued identification card before you can receive the ammunition. You must be 21 years old to get a FOID card in Illinois, which for all intents and purposes makes 21 the legal age to buy ammo.
There are several Illinois cities that greatly restrict ammunition sales to the point that we are unable to ship to their residents. There are also a number of local restrictions on magazine capacities.
On top of these state and local laws, all federal laws also apply.