Form Rule not firing off based on date field and formula field calculating age

I have a date field [Date of Birth] | a formula field that calculates the age [Age] & a dropdown field titled "Eligibility" (Eligible/Ineligible).

 The business rule is to change the Eligibility field to Ineligible if the Age is < 18. However, the form rule is not firing off the Eligibility field at all.