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WASHINGTON - In 2015, millennials became the biggest generation in the American workforce. 

Last year, they overtook Boomers as the biggest generation overall in the US At the same time, people are working later in their lives: Boomers aren't aging out of work, and seniors are employed at the highest rates in 55 years. 

The changing demographics have led to some tension at work for people both old and young. 

Seasoned employees suddenly have bosses half their age. 

Sometimes 25-year-olds with little experience are running entire teams, and in some cases, companies.