By ANDREA COOMBES

Growing numbers of older adults are finding a nice surprise in the workplace: a “Welcome” sign.

The number of workers age 55 and up grew by 3.5 million from September 2009 to September 2012. That represents the lion’s share of the gain of 4.2 million for all workers 16 and older, according to the Bureau of Labor Statistics. Two factors help explain the trend.

First: demographics. In the three years ended in July, 86% of population growth among people ages 25 to 69 came in the 55 to 69 age range, says Richard Johnson, director of the program on retirement policy at the Urban Institute, a nonprofit research group. That increase comes mostly from the baby boomers, who began turning 55 in 2001.

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“There are many more Americans turning 55 in recent years than turning 25,” Mr. Johnson says.

Second: changing attitudes. More employers are recognizing that older adults bring skills and experiences to the table that can help the bottom line.

It’s not all good news. While older workers’ unemployment rate is lower, when they lose a job they’re unemployed longer—a median of 35 weeks versus 26 weeks for younger folks.

“The problem of age bias hasn’t been solved yet, but attitudes do seem to be improving,” says Sara Rix, senior strategic policy adviser with the Public Policy Institute at AARP, the Washington advocacy group.

Here are several industries where experts say the outlook is bright for older workers:

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Education

School reform at the K-12 level, in particular, may provide opportunities for older workers, says Jackie Greaner, North America practice leader for talent management at consulting firm Towers Watson TW -1.04% .

“Expectations of teachers are much higher,” she says, “but in a way that provides opportunities for other talent to enter the school system—[individuals with] other types of skills and knowledge.”

Financial Services

“Banks and insurance companies have been forward-thinking about…the aging workforce and what that means for their organizations,” says Jacquelyn B. James, director of research at Boston College’s Sloan Center on Aging and Work.

“They’ve been trying to offer more possibilities to older workers to work more flexibly, to reduce their hours when they decide that’s what they want to do,” she says.

One example: Principal Financial PFG +0.92% offers a “Happy Returns” program to enable retirees to return to work without interrupting their benefits.

Health Care

Health-care companies are grappling with a shortage of workers and are reaching out to employees age 55-plus.

“There’s a huge need for people who can take care of all the different facets of health-care delivery, from the greeter at the door of the hospital to the person who processes accounts payable, and of course doctors, nurses and health technicians,” says Erin Peterson, senior vice president in the talent acquisitions solutions group at consulting firm Aon AON +0.29% Hewitt.

Among new health-care jobs identified in a 2010 report by Encore.org, a nonprofit advocacy group: patient advocates, community health workers and home-modification specialists.

Professional Services and Knowledge Workers

In the world of consulting, “it can be a plus to have experience,” says Ms. Greaner of Towers Watson. “There’s not really a stigma about being older.”

The same is true for other knowledge-worker jobs. For example, “the nuclear-power industry is an industry that is very hard to get people that are fully developed in terms of skill sets and capabilities,” Ms. Greaner says. For employers, “it’s very difficult to get that expertise.”

Aon Hewitt’s Ms. Peterson says talented recruiters can be hard to find. “I find people who have a lot of life experience and professional experience make the best recruiters.”

Ms. Coombes is a writer in San Francisco. She can be reached at next.

A version of this article appeared October 22, 2012, on page R4 in the U.S. edition of The Wall Street Journal, with the headline: For Older Workers, Here Is Where the Jobs Will Be.