We don't 0-index people's ages. There are a million books about "baby's first year", while they're still 0 years old.
In "figure of speech", or conventual use, people start drinking in their 21st year, not their 22nd. In common parlance, they can vote in their 18th year, not their 19th.
We talk of a child in their 10th year as being age 10. Might even be younger. Try asking a people if advice about a child in their "5th year of development" means you're dealing with a 5 year old. Most will say yes.
So perhaps it's logical to count from zero when there's no digit in the magnitude place, because you haven't achieved a full unit till you reach the need for the unit. Arguably a baby at 9 months isn't in their first year as they've experienced zero years yet!
Similarly "centuries" don't have a century digit until the 100s, which would make that the 1st century and just call time spans less than that "in the first hundred years" (same syllables anyway).
It's unsatisfying, but solves the off by one errors, one of the two hardest problems in computer science along with caching and naming things.
You are correct that nobody says "22nd year" in this context, but nobody says "21st year" either. The former is awkward but the latter is just incorrect.
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 completed being 17, but that's just too confusing, so people think you stop being a minor in your 18th year.
People commonly make the mistake of thinking otherwise, but that's all it is. A mistake.
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.
You sure?
“He was born in the summer of his 22nd year…”
That’s not the case, though. They can vote (and drink, in quite a few countries) when they are at least 18 years old, not when they are in their 18th year (who would even say that?)
People are 18 years old (meaning that 18 years passed since their date of birth) on their 18th birthday. There is no need of shoehorning 0-based indexing or anything like that.
> Most will say yes.
Most people say something stupid if you ask tricky questions, I am not sure this is a very strong argument. Have you seriously heard anybody talking about a child’s “5th year of development”, except maybe a paediatrician? We do talk about things like “3rd year of school” or “2nd year of college”, but with the expected (1-indexed) meaning.
> So perhaps it's logical to count from zero when there's no digit in the magnitude place, because you haven't achieved a full unit till you reach the need for the unit. Arguably a baby at 9 months isn't in their first year as they've experienced zero years yet!
It’s really not. To have experienced a full year, you need a year to have passed, which therefore has to be the first. I think that’s a cardinal versus ordinal confusion. The first year after an event is between the event itself and its first anniversary. I am not aware of any context in which this is not true, but obviously if you have examples I am happy to learn.
> It's unsatisfying, but solves the off by one errors, one of the two hardest problems in computer science along with caching and naming things.
Right. I know it is difficult to admit for some of us, but we are not computers and we do not work like computers (besides the fact that computers work just fine with 1-indexing). Some people would like it very much if we counted from 0, but that is not the case. It is more productive to understand how it works and why (and again cardinals and ordinals) than wishing it were different.
Writers.
And yes, cardinal versus ordinal is my point. The farther from the origin, the less people are likely to want them different.
Nevertheless the traditional "how old are you" system uses a number 1 less.
In Indian English terms, we do 'complete' age - aiui more common in India is to one-index, i.e. you're the age of the year you're in, and to disambiguate you might hear someone say they're '35 complete', meaning they have had 35 anniversaries of their birth (36 incomplete).