MM-DD-YYYY

// this code will run for form 4 only; change 4 to your form ID
add_filter('gform_validation_4', 'verify_minimum_age');
function verify_minimum_age($validation_result){

    // retrieve the $form
    $form = $validation_result['form'];

        // date of birth is submitted in field 7 in the format YYYY-MM-DD
        // change the 7 here to your field ID
        $dob = rgpost('input_7');

        // this the minimum age requirement we are validating
        $minimum_age = 18;

        // calculate age in years like a human, not a computer, based on the same birth date every year
        $age = date('Y') - substr($dob, 6, 4);
        if (strtotime(date('Y-m-d')) - strtotime(date('Y') . '-' . substr($dob, 0, 2) . '-' . substr($dob, 3, 2)) < 0) {
            $age--;
        }

    // is $age less than the $minimum_age?
    if( $age < $minimum_age ){

        // set the form validation to false if age is less than the minimum age
        $validation_result['is_valid'] = false;

        // find field with ID of 7 and mark it as failed validation
        foreach($form['fields'] as &$field){

            // NOTE: replace 7 with the field you would like to mark invalid
            if($field['id'] == '7'){
                $field['failed_validation'] = true;
                $field['validation_message'] = "Sorry, you must be at least $minimum_age years old. You're $age years old.";
                break;
            }

        }

    }
    // assign modified $form object back to the validation result
    $validation_result['form'] = $form;
    return $validation_result;
}

YYYY-MM-DD

// this code will run for form 4 only; change 4 to your form ID
add_filter('gform_validation_4', 'verify_minimum_age');
function verify_minimum_age($validation_result){

    // retrieve the $form
    $form = $validation_result['form'];

        // date of birth is submitted in field 7 in the format YYYY-MM-DD
        // change the 7 here to your field ID
        $dob = rgpost('input_7');

        // this the minimum age requirement we are validating
        $minimum_age = 18;

        // calculate age in years like a human, not a computer, based on the same birth date every year
        $age = date('Y') - substr($dob, 0, 4);
        if (strtotime(date('Y-m-d')) - strtotime(date('Y') . substr($dob, 4, 6)) < 0){
            $age--;
        }

    // is $age less than the $minimum_age?
    if( $age < $minimum_age ){

        // set the form validation to false if age is less than the minimum age
        $validation_result['is_valid'] = false;

        // find field with ID of 7 and mark it as failed validation
        foreach($form['fields'] as &$field){

            // NOTE: replace 7 with the field you would like to mark invalid
            if($field['id'] == '7'){
                $field['failed_validation'] = true;
                $field['validation_message'] = "Sorry, you must be at least $minimum_age years of age to join. You're $age years old.";
                break;
            }

        }

    }
    // assign modified $form object back to the validation result
    $validation_result['form'] = $form;
    return $validation_result;
}