Obviously the specifics vary by jurisdiction, but usually contracts that are 'necessary' (e.g. grocery store purchases) or beneficial to the minor (e.g. an employment agreement) cannot be voided simply because someone is under 18.

The further you go away from this line, e.g. a mortgage, the more likely a court of law would void the contract. As with many things in law, the specifics (if it makes to trial) is case-by-case and "it depends"; with settlement being generally based on a party's estimated chances of succeeding/costs should it go to trial.