Ben was 20 years old in 1980 but 15 years old in 1985 why is that?

The statement is a riddle rather than a literal fact. It suggests that Ben was 20 years old in 1980 B.C. (Before Christ) and 15 years old in 1985 A.D. (Anno Domini). The counting of years in B.C. decreases as you approach A.D., creating the apparent discrepancy in his age.