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 Montana, a contract created for an illegal purpose or against public policy is considered void and unenforceable by law. This means that neither party can seek legal recourse to enforce the terms of such a contract. On the other hand, contracts involving minors (typically under the age of 18), those based on mutual mistakes, or those formed under fraudulent misrepresentation are categorized as voidable. Voidable contracts can be disaffirmed or rejected by the disadvantaged party, such as a minor upon reaching the age of majority, or any party upon discovering the mistake or misrepresentation. However, these contracts can also be ratified if the disadvantaged party chooses to accept the contract terms after becoming aware of the contract's true nature. In Montana, the age of majority is 18, at which point a person gains the full legal capacity to enter into contracts.