Hyphenated ages

[Foolacious]

Has hyphenation of ages become the standard in all grammatical contexts?

When I was in school (60's and 70's), here is how ages were presented:

Rover is a 12-year-old dog.
Rover is 12 years old.

In other words, if the age functioned as an adjective, it was hyphenated; as a copulative predicate, it was not.

Now I seldom see, even in formal publications, the non-hyphenated form. Instead, it's:

Rover is a 12-year-old dog.
Rover is 12-years-old.

Oddly, I only rarely see this form extended to other measurements, like height or time.

Rover is a 30-centimeter-tall dog.
Rover is 30 centimeters tall.
RARELY: Rover is 30-centimeters-tall.

It was a 100-year war.
The war lasted 100 years.
RARELY: The war lasted 100-years.

Anyone have a copy of Chicago Manual of Style or other authoritative source handy?

[tblue37]

1. Grammar guru here. The "old" form is still the correct form.

Rover is a 12-year-old dog.
Rover is 12 years old.

[Blue_playwright]

8. Associated Press Style agrees!

[Blue_playwright]

9. Associated Press Style agrees!

[lettucebe]

2. Hypens in 12-years-old make no sense

I'm no expert but that is incorrect usage. Why add a hypen to twelve years? It is two words, twelve and years. They are two distinct things. Whereas Twelve-years-old is basically one word. It's stating a fact.

Also, if we want to talk numbers, always write out the number if it's the first word in a sentence. Do not start a sentence with a numeral. Just my thoughts.

[AmyStrange]

7. Except when it's a year...

-

2020 was a very bad year.
=======

[CrispyQ]

3. The younger writers in my group would agree that your first examples are correct.

However, when I read their work, I find that they often use en/em dashes in place of commas. And they cross out a lot of the commas I use in my writing.

[SheltieLover]

4. Perdue's Owl site is good

https://owl.purdue.edu/owl/english_as_a_second_language/esl_students/punctuation/hyphens_and_dashes.html#:~:text=Note%20that%20dashes%20are%20double,Apostrophes

I hope this helps.

[pjyonemura]

5. Apostrophe s

When I was in school and then an editor, apostrophe-s 's did not a plural make. That was in the 60s and 70s.

[Foolacious]

6. ON EDIT: Decades were presented to us as an exception...

and I've had enough pushback over the decades for "60s and 70s" that I've shifted over to "60's and 70's". With your encouragement, I shall endeavour to switch back!!

ON EDIT: It occurs to me that I was taught to pluralize letters by using an apostrophe: "My name is spelled with two r's."