Adjective order in academia: "Italian young adults"

I speak English only as a second language, and I don't do research that involves young adults, but my guess is that it's because "young adults" is a fixed expression that is very close to a compound word. So it feels weird to split it with an adjective. This would be a good question for English.SE, but I think that we are in the middle of a linguistic process where this expression is transitioning to become a true compound word.

Anyway, it is a common saying that "the international language of academia is broken English", so you shouldn't take academic texts as an example of style. For most academic writers English is a second (or third, fourth...) language, and many write by translating word-by-word constructs from their native language. It shouldn't be surprising to find the wrong word order.

Consider that the study is quite likely to be comparing young adults from a number of countries. The sentence "The Italian young adults have an average location further north the the Australian young adults" would work if you used "young Italian adults", but it would start to break down logically if you tried to avoid repetition "The Italian young adults have an average location further north the those from Australia" is fine but "The young Italian adults have an average location further north the those from Australia" seems to refer to people from both Italy and Australia.

The adjective order rule is a descriptive rule. When writing it can be treated as a rule of thumb at best. Consider the example in your link:

Have you ever wondered why we instinctively say “the shiny new red car” and not “the red new shiny car”?

Now the following conversations:

• "Which shiny new car is yours?" "The red shiny new car."
• "Which new car is yours?" "The shiny red new car."

Apart from the excessive repetition these are more natural orders because the important new adjectives come first.

In English, adjective order generally infers emphasis and grouping, rather than the adjectives being always equivalent and conforming to a standard order by type.

"Italian young adults" is probably making a distinction about the Italians amongst young adults.

"Young Italian adults" is probably making a distinction about the young amongst Italian adults.

Academic documents are probably more likely talking about young adult demographics, so the nationality is the more specific distinction and so that adjective is used first.

The above would be true even if (as other answers have pointed out) "young adult" were not a common demographic phrase.