(FORTUNE Magazine) – Now that baby-boomers outnumber their elders in the work force and in unions, the feisty striplings are questioning some of the most revered tenets of organized labor: seniority rules. In a dramatic sign of growing discontent, several hundred younger school service workers in Maryland's Montgomery County recently petitioned to abolish a seniority provision in their union contract. After the protest, Local 500 of the Service Employees International Union, which represents bus drivers, janitors, and cafeteria workers, among others, signed a new agreement that lets management choose the worker for a vacant position from among three qualified senior applicants. The previous system favored the most senior employee. Labor experts say a new breed of worker with higher aspirations is responding to a cutthroat work environment. ''Seniority is part of the religion of unions, but it's a concept that's breaking down all over,'' says Sar Levitan, director of the Center for Social Policy at George Washington University. ''We don't any longer have the kind of stability in our society that causes people to accept the rules that say they'll get ahead if they wait long enough.'' The median age of American workers has fallen from 39.2 in 1970 to 35.5 today. Only 28% of workers are over 44, creating a variety of pressures in the workplace. Says Richard Belous, a labor economist at the Conference Board: ''I < sense some intergenerational tension about how the compensation package is to be divided between cash now or pension money later.'' Others point to festering conflicts in work forces, such as those in the airline industry, where two-tier plans pay less to newer workers. The challenge to seniority rules may have less effect in manufacturing. Says Martin F. Payson, a New York labor lawyer: ''In occupations such as teaching and nursing, you can differentiate performance and individual contribution -- which is hard to do on the assembly line.''