"As a 19-year-old, with 'My Generation,' I wrote the most explicitly ageist song in rock," Townshend wrote on his band's website (www.thewho.com). The 1965 hit song includes the line "I hope I die before I get old."
"At 64, I now want to take on ageing and mortality, using the powerfully angry context of rock'n'roll."
"Floss" will be designed for outdoor performance and arenas and Townshend expects it to be ready for a concert premiere in 2011, probably in New York.
Some of the more "conventional" songs from the musical will appear on a The Who album set for release next year, he added.
"'Floss' is an ambitious new project for me, in the style of 'Tommy' and 'Quadrophenia,' " he wrote. "In this case the songs are interspersed with surround-sound 'soundscapes' featuring complex sound-effects and musical montages."
The story centres around a married couple whose relationship runs into difficulties.
Walter, a pub rock musician, finds sudden wealth when one of his songs is used in advertisements for a car company, but life turns sour when he returns to music after a 15-year break.
"While Roger Daltrey exercises his ageing vocal chords by embarking on a two-month 'Use or Lose It' solo tour, my focus is on 'Floss,' which touches on the current issues faced by the Boomer generation," Townshend wrote, referring to The Who's lead singer.
"It also addresses their uneasy relationship with their parents, children and grandchildren."