When creating an age determination date fast formula, I need to be able to determine the age of the

I recently was advised that starting next year, two of my three dependent types when turning age 26 need to retain their coverage thru the end of the month.  Yes, I can run their lose dependent eligibility life event on the last day of the month but I am concerned about intervening life events that could end date the coverage early.  I was hoping to use an age determination date fast formula in my derived factor so that unless it was the lose dependent eligibility life event that it would use the last day of the previous month (but then