It has been asked (almost the same question), I think that "irati" agrees with young men, not as an adverb, but like a noun modifier (=an adjective) for young men.
How to form an adverb in Latin?
"In Latin, some adverbs are formed by adding an ending to an adjective.
For first and second declension adjectives, a long -e replaces the ending.
Instead of the adjective carus, -a, -um (dear), the adverb is care. To adjectives from the third declension, -ter is added. From the adjective fortis 'brave', the adverb form is fortiter.
The neuter accusative of some adjectives is also the adverb. Multum 'many' becomes multum 'much' as an adverb.
The formation of other adverbs is more complicated."
If you refer to the fact that an English adjective could be used like an adverb, I think it's not the case in Latin, because, in English, you would understand it's used in an adverbial way, from the word order. "The angry young men", because of the order, always mean in any sentence, "the angry young men", and never "angry" like an adverb qualifying the verb.
So, I don't think it could be possible in Latin, especially when "irati" is plural like "iuvenes", it would make no sense to agree it to the plural, if it was used as an adverb.
No one would claim that it is not an adjective. It clearly is. But adjectives, in Latin as in English, can be used with an adverbial function. I understand your reasoning, but as a beginner to the language you are reasoning in a vacuum of knowledge of the language's actual practice: it IS common in Latin for an adjective to be used where in English we would more naturally use an adverb.
An example. Here Sallust is talking of the senate. The sentence starts this way: "Ceterum supplementum etiam laetus decreverat", "Besides this, it had even happily granted him an increase in troops". Of course "laetus" is an adjective, agreeing with an implied "senatus", but what hope is there for someone insisting on forcing it to work as an adjective here? "An even happy it had granted..."? "Even happy had it granted..."? "It had granted, even happy, ..."? Not improvements, I think.
Exactly. The subject is described, but the force is on the verb. The adjective is predicative, not attributive (not the angry youths, but the youths, being angry...); in English it is more natural to describe the verb in such cases.
That is possible, although “the young” tends to refer to young mammals in general. With your sentence I thought of lion cubs, not humans.