n. Injuries to older, amateur athletes, especially those who are part of the Baby Boom demographic.
Forty-four-year-old Rick Gardner admits he did all the wrong things. Didn't change into sneakers. Wasn't wearing his leg braces. Hadn't played in a while. The consequences of all that didn't become clear, though, until he went for the long rebound and felt the excruciating pop in his knee. Yep, he'd blown it out. Gardner, who lives in Norfolk, didn't know it at the time, but he also was suffering from a larger societal ailment dubbed 'Boomeritis.'
—Elizabeth Simpson, “Banged-up boomers,” The Virginian-Pilot, July 26, 2001
1999 (earliest)
Dr. Nicholas A DiNubile has seen enough baby boomers with exercise injuries to come up with a name: 'boomeritis.'

'There is a mini-explosion of these injuries,' said DiNubile, an orthopedic surgeon at the Hospital of the University of Pennsylvania, Philadelphia.
—Ira Dreyfuss, “'Boomeritis' afflicts aging, exercising boomers,” The Associated Press, October 24, 1999
As our New Year's resolutions kick in, those of us of a certain age should view the first citation as a cautionary tale, particularly if we've resolved to make exercise a regular part of our lives. Just remember that our innards — especially our joints, ligaments, and tendons — aren't what they used to be.

In an auspicious beginning to the Word Spy year, the earliest citation I could find for this word is also kind enough to tell us the name of the person who coined it.