An attempted contract that is made for an illegal purpose or that is against public policy cannot be enforced by either party and is void. In contrast, a contract that is made by a minor (usually under age 18), or that is based on a mutual mistake, or that was made on the basis of a fraudulent misrepresentation by one party, is voidable and may be rejected by a party who was at a disadvantage when the contract was made. A voidable contract may also be ratified and accepted by such a party—after a child reaches the age of majority (usually 18), or the parties become aware of their mutual mistake, or a party learns of the other party’s misrepresentation and proceeds with performance of the contract.

In Illinois, contracts made for illegal purposes or against public policy are considered void and unenforceable. This aligns with the general principle of contract law that prohibits the enforcement of agreements that involve illegal acts or that contravene societal standards. On the other hand, contracts involving minors, mutual mistakes, or fraudulent misrepresentations are categorized as voidable. This means that the disadvantaged party has the option to either reject or ratify the contract. For instance, a minor in Illinois can disaffirm a contract before reaching the age of majority (18 years old) or within a reasonable time afterwards, but they can also choose to ratify the contract upon reaching adulthood. Similarly, parties to a contract based on a mutual mistake or fraudulent misrepresentation can either rescind the contract upon discovery of the issue or may choose to proceed with the contract, thereby ratifying it. It's important to note that specific statutes and case law can modify these general principles, and an attorney can provide detailed guidance based on the particular circumstances of a contract dispute.