The guy who developed the way we reckon years was a 6th-century monk named Dionysius Exiguus (“Dennis the Short”). He apparently thought Christ was born in 1 B.C. (actually, it's a bit more complex than that, but we'll keep this simple). read more
Jesus would be about 2,020 years old as of 2017 instead of 2021--that's because of a quirk in the Christian calendar in which there is no year zero (it goes directly from 1 BC to AD 1). But Jesus may be as old as 2024. read more