Where does “sedmdesátiletý” come from?

sedmdesátiletý (Czech) comes from Czech sedmdesát, from Czech deset, from Old Czech desět, from Proto-Slavic desętь, from Proto-Balto-Slavic *déśimt, from Proto-Indo-European déḱm̥t — ten.

sedmdesátiletý (Czech): seventy-year-old