Earlier quoted context omitted.
> nobody says "21st year" either On the contrary, enough people say it, it's a quora question: https://www.quora.com/What-does-it-mean-to-be-in-your-twenty... Authors love phrases like this. Which, in turn, comes from another ordinal/cardinal confusion stemming back to common law: "A person who has completed the eighteenth year of age has reached majority; below this age, a person is a minor." That means they complet…
It's just not true. You've completed being 17 years old on your 18th birthday, when you enter your 19th year and can count 18 years under your belt. Consider a newborn. As soon as they're squeezed out they are in their first year of life. That continues until the first anniversary of their decanting, at which point they are one year old and enter their second year of life. There is nobody, nobody, who refers to a bab…
No one's saying it's true! All that's being claimed is that writers will often use phrases like "became an adult in their 18th year" or "was legally allowed to drink in their 21st year".
It's completely incorrect, but some people use it that way, and ultimately everyone understands what they actually mean.