The workers brought the suit after discovering that nearly half — or 164 — of those in the 2013 layoffs were 40 or older, the age that initiates federal age discrimination law protections. And workers charge that they were singled out, in addition, because either they or their spouses had serious medical conditions.

Spirit maintains that it does not discriminate in hiring or termination decisions.

“Reductions in force are never easy, however all decisions are based on job-related, nondiscriminatory criteria,” said Fred Malley, Spirit’s spokesman.

“We are confident the evidence in this case will show Spirit is compliant with the law in its employment practices.”

Such lawsuits are popping up as the nation’s work force ages and as many longtime workers claim that they are being deliberately targeted for such reductions. As manufacturing has contracted, more experienced workers feel they have limited options for re-employment if they are discarded at older ages.

“Once layoffs were done by reverse seniority. It was last in, first out, so the more senior workers kept their jobs,” said Robert J. Gordon, an economics professor at Northwestern University, who studies the country’s growth and work force productivity.

“Now we’re seeing a transition from the age of favoritism to that of age discrimination,” Mr. Gordon said, “because newer workers are allowed to stay on while more costly, older workers are let go.”

One of the few recourses for employees is to file a job discrimination complaint with the Equal Employment Opportunity Commission. Nearly 21,000 age discrimination complaints were filed in 2016 with the commission, up from 20,144 in 2015, though down slightly from a high of almost 25,000 in 2009 during the financial crisis, when huge numbers of jobs were eliminated.