That's not exactly true. You don't have to have a card if u r under direct control of a card holder. Ie father and son hunting in same blind.

I am under the age of 21, can I apply for a FOID card? Yes. In addition to all other requirements, a person who is under 21 years of age must have the written consent of his or her parent or legal guardian to possess and acquire firearmsand firearm ammunition. Also,heor shemust not have been convicted of a misdemeanorotherthan a trafficoffenseor adjudged delinquent. The parent or legal guardian providing written consent must be eligible to possess a valid FOID card.
I am under the age of 21, do I need a FOID card? Not necessarily. The FOID Act exempts unemancipated minors while in the custody and immediatecontrol of their parent, legal guardian, or other person in loco parentis to the minor as long as their parent, legal guardian, or other person in loco parentis to the minor has a valid FOID card in their possession.