This code will enable the question if the person is over 17 as of the effective date of the plan.
 
You have to use Event.Engine.EffectiveDate and NOT Event.Application.EffectiveDate because Event.Application.EffectiveDate is not given a value until later in the application process. 

Event.Value = (Event.CurrentPerson.Age(Event.Engine.EffectiveDate) > 17);