To add simple condition to the field (i.e. Date_of_birth<[now]):

• Double click on the corresponding field in the main form;
• In the rule builder tree right click on the Rules node and select Add->Value:
  
• In the pop-up control select the operator and value (leave blank if condition is IS NULL or IS NOT NULL);
• Click on the Y button in the right-bottom corner of the control;
  
• Press on the OK button of the Field Details form to get back to the main form;