On his 65th birthday, physical therapist Gary Kinsey incorporated a company that makes medical devices.

“It is a huge risk,” said Kinsey, now 67, owner of North Florida Medical Solutions Inc. in Gainesville. “I have sunk everything I have into it the last two years. I have put myself out on the line.”

Older Americans such as Kinsey are increasingly shunning retirement to start companies because they see job opportunities limited after age 55, don’t have enough savings to retire comfortably or want to work for themselves. People from ages 55 to 64 started 23.4 percent of companies in 2012, up from 14.3 percent of new entrepreneurs in 1996, according to the Kauffman Foundation’s research.

“The 30-year corporate job with a gold watch isn’t there anymore,” said Dane Stangler, vice president of research and policy for Kansas City, Mo.-based Kauffman. “A lot of people are not ready to retire. We are living healthier for longer, and they are looking for a main income or a supplementary income.”

Mark Zuckerberg started Facebook as a college student at Harvard University, but experienced managers may be better prepared for startups, said Jeff Mariola, 59, who in October started Chicago-based Digital BrandWorks, which acquired an electronic commerce company, www.buyhappier.com.

“I have always wanted to own my own business. It is your baby,” said Mariola, whose company has 18 employees and offers Internet consulting services to manufacturers. He says it may employ 150 people in four years.

He’s drawing on his experience as president of Ambius, having managed a unit with more than 1,200 employees. Ambius provides art and plants to offices and is a division of Rentokil Initial, a facilities manager based near London.

“Having experience is critical,” Mariola said. “Banks are generally conservative and want to be sure management teams understand cash flow and balance sheets, the things that underpin a business.”

More startups are on the way. About a quarter of Americans between ages 44 and 70 are interested in creating their own company or nonprofit venture, according to November 2011 research by San Francisco-based Encore.org, which studies baby boomers’ plans for new careers.

The average age of 500 recent applicants for a Florida entrepreneurship program funded in part by the U.S. Labor Department was 51, said Michael O’Donnell, regional project manager for Startup Quest in Fort Lauderdale.

“People over 50 are seriously exploring startups as an option,” he said. “These are highly educated people that were laid off in the last downturn and realize those jobs are not coming back.”

Florida, a retirement destination, has been attracting older people who migrate to form businesses, said Andrew Duffell, chief executive officer of the Research Park at Florida Atlantic University in Boca Raton. The park includes a technology business incubator to assist startup companies, with “a good 30 percent” run by those over 50.

The incubator includes companies that develop software, manage logistics for transportation of products such as medical devices, and do advanced engineering.

The 55 and older Americans may have more confidence in their abilities than prior generations, demographers said.

“Baby boomers are more educated and probably more entrepreneurial than earlier generations,” said William Frey, a senior fellow at Brookings Institution in Washington.

They are more likely to own their own homes and have some assets that can be used to finance businesses, said Kenneth Johnson, senior demographer at the Carsey Institute at the University of New Hampshire.

Joseph Schmoke, 69, ran a for-profit college in Birmingham, Ala., until 2010, and was dismayed that smaller colleges such as his were typically ignored by ratings services.

In 2011, he founded University Research & Review, which has two websites that help students select from among smaller colleges and now has four full-time and seven part-time employees.

“I am tired of boating,” said Schmoke, who started his business in the Boca Raton research park. “I don’t play golf. I am in good health. I want and have the energy and need to do something. We can have a positive effect and probably make a little money too.”