Merriam-Webster Collegiate Dictionary - sixty

 noun  (plural sixties)  Etymology: Middle English, from ~, adjective, from Old English siextig, noun, group of ~, from siex six + -tig group of ten; akin to Old English tien ten  Date: 14th century  1. — see number table  2. plural the numbers 60 to 69; specifically the years 60 to 69 in a lifetime or century  • sixtieth adjective or noun  • ~ adjective  • ~ pronoun, plural in construction  • ~ish adjective