Twifties are the fun people around age 50 and up. More Magazine printed the following definition in the May 2010 magazine: Twifties (plural noun) People over 40 who act like they're in their twenties.
The word was created in 2008 by professional naming expert Lauren Teton in NY and derivation is "Fifty is the new Twenty. Twenty, Fifty, ...Twifty." Teton felt a word was needed to describe the most active, vital subset of the Baby Boomers and adults older than Boomers.