You can use validation groups for this:
@Min(value = 18, message = "You have to be 18", groups = AdultsGroup.class)
public int age;
and then :
constraintViolations = validator.validate( person, AdultsGroup.class );
for checking min age, or:
constraintViolations = validator.validate( person, null );
for not checking this constraint.
You can read more details here: http://beanvalidation.org/1.1/spec/#constraintdeclarationvalidationprocess-groupsequence