I think this falls firmly in the badass category: a family of four (the two kids are 7 and 9) have nearly completed a thru-hike of the AT. Their chronicle is at
http://kallinfamily.com/. They aren't early retirees, but I'd say they share many of the attributes of Mustachianism: lead a good life, enjoy life, do things your way, and don't waste.
Anyway, their chronicle is pretty inspiring.