The National Institute on Aging says 12-14 points, but I'd like know what makes sense in pixels since point size are different for different font families. Google Bootstrap body font is 14px. Is that adequate for older adults?
The default size has always been at 16px. I would highly recommend using 16px on all sites. 14px is the smallest I would go for body.
12px is extremely small on a desktop. It was mainly used for aesthetics, but most sites should have abaondoned this practice.
here's a tool to help for conversions. http://pxtoem.com/
Article explaining why 16px is standard http://www.smashingmagazine.com/2011/10/07/16-pixels-body-copy-anything-less-costly-mistake/
I would like to assume 16px is a good start for senior citizens. Considering the fact that 16px is comparable to reading a book from the conventional distance. Of course there are people who needs glasses to read. From my experience the older groups in our tests struggle at 14px, but not at 16px. If they need bigger fonts, I would maybe try to suggest the user to ctrl/apple +/- before the content is served.