Capacity

-An individuals ability (in the eyes of the law) to enter into a legally binding relationship

-some people do not have the requisite intent to enter into an agreement ie.
    Minors
    Mentally impaired

this is because they don't have the understanding necessary to enter into a binding agreement

In common law, people used to have to be 21 to have capacity to contract, but now the majority is 18

Void - never came into existance

Voidable - Valid, but subject to cancellation

in common law the exception to this rule was contracts for necessaries like food, shelter, or clothing.

deal with a minor at your own risk

Mentally incompetent vs officially mentally incompetent

If a court has officially determined that an individual is incompetent then all contracts are void
if something is void in the law it can never be contracted

a mentally incompetent has a reasonable time once they become competent to void a contract

Minors can Ratify an agreement
ratify means recognize and acknowledges the existence of the agreement

upon reaching the age of majority any minor recognizes and acknowledges the existence of the agreement it can not be voided and has therefore been ratified.