Why do we add the plural suffix to alacsony but not to fiatal?
No endings on adjectives that are before nouns (like "fiatal" in this sentence).
Linguistically speaking: predicative adjectives inflect but attributive adjectives do not.
Oh, that makes sense, thank you!
This is just how it's done. Fiatal fiúk means young boys, in which case fiatal is not pluralized. Alacsonyak means "they are short" here, in which case you do pluralize.