Knowing you should stay active for the sake of your physical health during retirement may have inspired you to take a Tai Chi class or join a nature walk group. If you want to boost your brain health, online classes for seniors might be a good prescription.

Research suggests that learning in retirement may improve your cognitive health — the ability to clearly think, learn and remember.

You can find free online classes in a seemingly endless array of subjects. Some sites offer classes targeted toward older adults, while others were built for a more general audience. For a sampling of what's available, check out the classes offered through these five sites.

1. edX

One of the largest selections of online classes is available at edX, which offers more than 3,000 courses from 160+ member universities, companies and other organizations worldwide. Pick your subject from among topics including art, music, philosophy, foreign languages, science, business and coding.

Take a course from the University of Pennsylvania on the history of Hollywood, explore philosophy and critical thinking in a course from Australia's University of Queensland, or delve into the health effects of climate change with a Harvard professor. Online discussion forums let you connect with your fellow students from around the world.

2. Coursera

With more than 1,700 free courses from hundreds of leading universities and businesses around the globe, Coursera offers plenty of online classes for seniors and other learning enthusiasts.

Follow your curiosity into some of the most popular courses, such as "The Science of Well-Being" from Yale University, "Finding Purpose and Meaning in Life" from the University of Michigan, or "Learning How To Learn: Powerful Mental Tools To Help You Master Tough Subjects" from Canada's McMaster University in Hamilton, Ontario.

Classes feature video lectures, community discussion forums and assignments that are graded automatically and peer-reviewed. For a small fee, you can obtain an electronic course certificate after completing each class.

3. Duolingo

Say you're planning a vacation to Italy and want to speak the local lingo with the new friends you'll meet. Or maybe you're about to welcome a new granddaughter from Korea into the family, so you'd like to learn some of her native language. Duolingo, a free online language learning platform, offers a fun way to become fluent in more than 30 languages.

Using five-minute, "bite-sized" lessons, the site aims to make learning a new language feel like a game. Rather than drilling you on language rules, Duolingo uses an approach called implicit learning that allows you to discover language patterns naturally. The site's high-tech tools help you learn at your own pace: As you work on each lesson, machine learning algorithms determine which level of difficulty you're ready for next.

4. AARP Virtual Community Center

AARP launched its Virtual Community Center as a member benefit in February 2021. The site offers a variety of free online classes for seniors, including programs from the nonprofit Older Adults Technology Services. The summer 2021 schedule includes offerings like virtual chair yoga, a presentation from the New York Historical Society on U.S. presidents, an introduction to social media, a virtual wine tasting class and a summer nutrition cooking series.

5. Codecademy

If you enjoy working with computers and want to try learning coding, Codecademy can help you finally achieve that goal now that you're enjoying a bit more leisure time in your retirement. The nine-hour course "Learn HTML," for example, teaches the most widely used language for building websites.

You can also learn additional programming languages as well as web design. Codecademy claims that no matter what your level of experience, you'll be able to write working code just minutes into one of its classes.

Apart from its benefits for cognitive health, learning in retirement can also boost happiness. Exercising your brain is simply good for your well-being — taking an online class for seniors can breathe new life into old pastimes or jump-start a new fascination.