Generation XL
n. Children or young adults who are overweight.
Overweight children trudge through their school years on the way to an adult life rife with health problems. They are the victims of an epidemic of childhood obesity in the United States, and yet public policy makers are tiptoeing around one of the root causes of their disease: too much eating of the wrong kinds of foods. … [T]he percentage of overweight young people has increased from 4 percent in 1963 to 15 percent in 1999.
—“Generation XL; An obesity epidemic,” The Boston Globe, May 27, 2003
Internet workers call it "the start-up 15," the extra pounds they gained when they gave up a balanced diet and regular exercise for their dot-com jobs.

"In my case, it's the start-up 24," Phillips said.

Meet Generation XL. Like college freshmen who get fat from too much dorm food and too little activity, many cubicle potatoes lead very unhealthy lives. They have erratic eating habits and indulge in too much late-afternoon or late-night high-fat snacking. They are only half joking when they say their only physical activity is surfing the Internet.
—Jessica Guynn, “As dot-coms trim fat, so do ex-workers,” Contra Costa Times, March 19, 2001
1995 (earliest)
If researchers are correct that people in their 20s today - the so-called Generation X - are heavier and less physically active than people in that age group five to 10 years ago, that would make them Generation XL, wouldn't it?
—Bob Molinaro, “Speeding up baseball,” The Virginian-Pilot (Norfolk), July 14, 1995
Thanks to Olin Sibert, who told me about the Boston Globe citation, which is part of a series titled "Generation XL" that focuses on child obesity.