How much younger is substantially younger in an Age case?

As I have written about before you can prove an age discrimination case by circumstantial evidence if you can prove that you were replaced by a substantially younger individual and that there was no reason or a false reason for your termination.  But the question that always arises is what is substantially younger?  The 5th Cir in Flanner v. Chase Investment Services Corp., 2015 U.S. App. LEXIS 1603 (5th Cir. 2015) (unpublished) held that 5 years younger could be substantially younger.  This case is interesting because the 59 year old employee’s responsibilities were actually split between an employee who was 5 years younger and one who was 27 years younger.  But the 5th Cir held that 5 years could be substantially younger for the purposes of age discrimination.  Many people working in El Paso, Texas and Las Cruces, New Mexico will benefit from this opinion because age discrimination is a growing problem with an aging population.  This seems to be more pronounced in El Paso, Texas and Las Cruces, New Mexico because we have many younger workers who are available to take the jobs of older El Paso, Texas workers.