how to validate minimum age in webform
You can do it using the hook_webform_submission_presave() exposed by the Webform api in a custom module. 

Another alternative is to make use of this sandbox project


  This module provides a way to calculate values for Webform components
  and/or to add custom validation code without the need for creating
  custom modules that implement hooks. It offers a PHP-like macro
  scripting language to use instead.
  
  Differs from Webform PHP in that there is no eval() evil and macro
  code has no direct access to Drupal environment or functions, i.e.
  macro code is contextually jailed.
  
  Differs also from Webform Calculator in that it permits logic-driven
  calculations and also integrates with the Webform Validation module.