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

If(Today()>ToDate(ToText(Month([Date of Birth]))&"-"&ToText(Day([Date of Birth]))&"-"&ToText(Year(Today()))),
Year(Today())-Year([Date of Birth]),(Year(Today())-(Year([Date of Birth])))-1
)

This is not a formula date, it is a formula number. Change your data type for this field to formula number. Other than that, the formula is fine.

Doing this will fire off the form rule for the "Eligibility field"???

The formula that calculates the age is working just fine.

The [age] is already a formula number field

The eligibility checkbox should be a formula field and not a form rule.  Form rules do  not "fire" with the passage of time.  They only fire when a record is edited by a human user.  But, we all get a day older every day just through the passage of time. 

Sure, however, this forms' data is static, it is an application. The eligibility should not change over time if the age becomes greater than or equal to 18.

formula check box field:

if([formula age calc field]>=[Eligibility Age],true,false)