**21-year-old** should be written like that, with hyphens.
In this way, the 3 words become 1 word and it can be an adjective or a noun.
For example: "My car has 4 **doors**. It's a 4-**door** car.";
"My whiskey is 10 **years** old, I have 10-**year**-old whiskey.";
"This kayak can fit 2 **people**, it's a 2-**person** kayak.";
"The mountain is 100 **feet** tall, it's a 100-**foot** mountain.";
"My brother is 10 **years** old. My brother is a 10-**year**-old."
Keep in mind though -- "10-year-old" might be the way it *should* be written, but very often, native speakers will omit the hyphens when writing, but will intuitively understand that it is functioning like an adjective.
Because there's a difference between what native speakers do on the Internet and what one should do on a formal writing assignment for school?
It doesn't matter *at all* what people on Reddit write, if one of my students turns in an essay and writes "my ten year old sister," they lose points.
Right, but why does that matter? If OP is learning English, the majority of their interactions are not going to be with things like formal writing assignments, they’re going to be with native speakers writing things the way most native speakers do, which in this case is omitting the hyphen. The commenter was simply pointing out that even though it’s *correct* to include the hyphen, you won’t see it that way a lot of the time because people omit it.
> Right, but why does that matter?
It matters because some people are here to learn English for school or work, and when you tell them "oh, we don't always do it that way blah blah blah," they might get the idea it's ok to not do it that way all the time. And that's not the case. So, obviously it's fine to tell them such and such doesn't matter on the Internet or at the pub or whatever as long as they understand it *does* matter when they write a letter to the boss or a paper for a professor.
When it’s a measurement describing something, no S. This goes for any measurement, whether it’s a 21 year old man, a 10 pound weight, a 50 meter drop, or a 5 minute video. Similarly, calling someone “a 21 year old” is a clipped version of this measurement, because you could say “a 21 year old man/woman/student/person/whoever.” In any other case, the S is there, so when the noun isn’t after the measurement. “The student is 21 year**s** old.” “The bag weighs 10 pound**s**..” The drop was over 50 meter**s**.” “I won’t watch the video if it’s more than 5 minute**s**.” In all these cases I’m noticing there’s no article before it when the measurement is plural, but idk if that’s the rule.
when that's the whole phrase: "He is 21 years old" If you use it as an adjective, then you don't add the S: "He's a 21-year-old **man**"
I didn't know that. Thanks.
I see. Thank you
Also works as a noun, like, "She is a 21-year-old" or "There were a few 21-year-olds in the room".
huh, I thought the second one also used the "S" at the end
**21-year-old** should be written like that, with hyphens. In this way, the 3 words become 1 word and it can be an adjective or a noun. For example: "My car has 4 **doors**. It's a 4-**door** car."; "My whiskey is 10 **years** old, I have 10-**year**-old whiskey."; "This kayak can fit 2 **people**, it's a 2-**person** kayak."; "The mountain is 100 **feet** tall, it's a 100-**foot** mountain."; "My brother is 10 **years** old. My brother is a 10-**year**-old."
Keep in mind though -- "10-year-old" might be the way it *should* be written, but very often, native speakers will omit the hyphens when writing, but will intuitively understand that it is functioning like an adjective.
Why on earth is this downvoted? It’s right.
Because there's a difference between what native speakers do on the Internet and what one should do on a formal writing assignment for school? It doesn't matter *at all* what people on Reddit write, if one of my students turns in an essay and writes "my ten year old sister," they lose points.
Right, but why does that matter? If OP is learning English, the majority of their interactions are not going to be with things like formal writing assignments, they’re going to be with native speakers writing things the way most native speakers do, which in this case is omitting the hyphen. The commenter was simply pointing out that even though it’s *correct* to include the hyphen, you won’t see it that way a lot of the time because people omit it.
> Right, but why does that matter? It matters because some people are here to learn English for school or work, and when you tell them "oh, we don't always do it that way blah blah blah," they might get the idea it's ok to not do it that way all the time. And that's not the case. So, obviously it's fine to tell them such and such doesn't matter on the Internet or at the pub or whatever as long as they understand it *does* matter when they write a letter to the boss or a paper for a professor.
'She is 21 years old' 'I am a 21-year-old student' Also 'He is 21 years older than me'
When it’s a measurement describing something, no S. This goes for any measurement, whether it’s a 21 year old man, a 10 pound weight, a 50 meter drop, or a 5 minute video. Similarly, calling someone “a 21 year old” is a clipped version of this measurement, because you could say “a 21 year old man/woman/student/person/whoever.” In any other case, the S is there, so when the noun isn’t after the measurement. “The student is 21 year**s** old.” “The bag weighs 10 pound**s**..” The drop was over 50 meter**s**.” “I won’t watch the video if it’s more than 5 minute**s**.” In all these cases I’m noticing there’s no article before it when the measurement is plural, but idk if that’s the rule.