After a night on the tiles, you discover you've written a cheque to a friend of a friend to buy his pet shark. Fortunately for you, contracts made by drunkards aren't enforceable. Cancel your cheque immediately. Contracts made by people under 20 years old are governed by the Minors Contract Act, unless they are or have been married. Minors who are 18 or 19 years of age are normally bound by contracts, unless the consideration was inadequate or the terms of the contract were harsh or oppressive on the minor.
All minors can enter into binding apprenticeship and life insurance contracts.
And grown-ups beware. Even though a contract may not be enforced against a minor, a minor can enforce it against an adult. I cancelled my hotel booking. You breached your contract with the hotel. The hotel could enforce the contract by insisting that you pay the full price of the stay you arranged.