Baby boomers are retiring in droves. Here are three big reasons for concern.

The youngest baby boomers are around 55 years old. The oldest are in their 70s. Most Americans don’t remember a workforce without the largest generation.

And yet, as boomers enter their final years in the workforce, their retirements are taking companies by surprise.

In the next five years, almost three-quarters of the companies surveyed in 2018 by Willis Towers Watson, a risk-management and insurance brokerage company, expect to face significant or moderate challenges from late retirements. But because nothing is predictable, a significant share are also worried about early ones.

Companies typically said they were more concerned than they used to be about the cost of older workers, and the challenge of replacing the knowledge and skills those older workers will take with them on the way out the door. The reported rise in concern over older workers “blocking promotions of younger employees” was not as sharp, though about two-thirds of respondents said it was at least a moderate concern.

That tends to track somewhat with each sectors’ reliance on older workers, but individual industries within each sector vary considerably. Funeral homes, religious organizations, bus drivers, florists and real estate agents all have more than a third of their workers over age 55, according to our analysis of the Census Bureau’s American Community Survey.

At the other end are industries that tend to rely on a younger workforce, such as retail, restaurants, electronics stores, theaters and warehouses. In those fields, fewer than 1 in 7 workers are nearing retirement.

In many industries, employers also face significant uncertainty as to whom or what will replace a retiring boomer, Glickstein said.

Respondents reported that “loss of specific company knowledge” would be an increasingly difficult challenge, as would “finding workers with similar knowledge and skills.”

As technology progresses, it’s feasible that a robot or computer program could take on some of a worker’s responsibilities when he or she is ready to retire. That means employers’ succession strategies will evolve alongside technology.

And because boomers are the first generation to face the task of passing on their knowledge to machines and computer programs — as well as to apprentices and junior employees — setbacks and tensions are bound to arise.

“How do you translate that business knowledge into an algorithm?” Glickstein asked.

The Washington Post’s Scott Clement contributed.

This story was first published by The Washington Post.