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 baby as being in their zeroth year of life. Nor would they refer to a one-year-old as still being in their first year of life as if they failed a grade and are being held back.
The pattern continues for other countable things. Breakfast is not widely considered the zeroth meal of the day. Neil Armstrong has never been considered the zeroth man on the moon nor is Buzz Aldrin the first. The gold medal in the Olympics is not awarded for coming in zeroth place.
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.
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 baby as being in their zeroth year of life. Nor would they refer to a one-year-old as still being in their first year of life as if they failed a grade and are being held back.
The pattern continues for other countable things. Breakfast is not widely considered the zeroth meal of the day. Neil Armstrong has never been considered the zeroth man on the moon nor is Buzz Aldrin the first. The gold medal in the Olympics is not awarded for coming in zeroth place.