Like this:
validates_date :date_of_birth,
:unless => 'date_of_birth.blank?',
:before => lambda { 6.years.ago },
:before_message => 'must be at least 6 years old',
:on_or_after => lambda { 70.years.ago },
:on_or_after_message => 'please enter a valid date of birth'
And you probably shouldn't block people over 70 years old like that :)