These cities boast easy-to-understand tax regimes and helpful networking programs.

One in every five American workers is over age 65 in 2015, and by 2020, one in four workers will be over age 55, according to the U.S. Bureau of Labor Statistics. As people live longer -- one in four people who reach 65 will live past 90 -- it's not surprising that they are working longer. But rather than work for someone else, many boomers are embracing entrepreneurship, starting their own businesses as they seek second careers that will give them flexibility and allow them to pursue new passions.

So where are all these older people working? Thumbtack, an online marketplace that matches service providers with consumers, has the answer. The company released the results of a survey today that analyzed 1,000 responses from small business owners over 65 to determine which U.S. cities are friendliest towards senior entrepreneurs.

After rating cities on factors that were most important to them -- tax regimes, licensing rules and the presence of helpful training and networking programs, among others -- Portland, Oregon claimed the top spot.

In a similar survey released by Thumbtack last month, in which cities were ranked based on their friendliness towards millennial entrepreneurs, Birmingham, Alabama earned top billing.

Check out the complete list of friendliest cities towards seniors below. To explore more data from the survey, go here.

10. West Palm Beach, Florida
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9. Washington, D.C.
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8. Seattle, Washington
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7. Phoenix, Arizona
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6. Minneapolis, Minnesota
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5. Denver, Colorado
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4. Dallas, Texas
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3. Atlanta, Georgia
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2. Houston, Texas
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1. Portland, Oregon
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