AOL article highlights some items to watch for. https://www.aol.com/2011/05/17/top-signs-of-age-discrimination/
Link to the in-depth IBM study regarding their initiative to "correct their seniority mix". https://features.propublica.org/ibm/ibm-age-discrimination-american-workers/
EEOC homepage - https://www.eeoc.gov
Finally, even if your name doesn't come up on this round, you should start documenting everything as a personal insurance policy.