Your metaphor is comparing apples and oranges. When we could life, it's "one year old" or "aged one year," both of which mark the completion of a milestone. Usi

Your metaphor is comparing apples and oranges. When we could life, it's "one year old" or "aged one year," both of which mark the completion of a milestone. Using the term "18th century" is all-encompassing of that year, which is a different use case. When one recollects over the course of someone's life, like in a memoir, it would be normal to say "in my 21st year", referring to the time between turn 20 years old and 21 years old.