https://www.bizjournals.com/chicago/news/2018/03/08/united-airlines-loses-age-discrimination-lawsuit.html
Here's a link for UAL's unsuccessful motion to dismiss the above case:
https://www.leagle.com/decision/infdco20160412932
A caveat is that these employees did have a collective bargaining agreement in place.
Resources to consider in Illinois (where suit must be presumably filed):
STOWELL & FRIEDMAN, LTD. 312-431-0888
https://attorneys.superlawyers.com/discrimination/illinois/chicago/