It cannot be "Şunlar köpekler yaşlı." (I'll explain why in a moment)
- Şu köpekler yaşlı = Those dogs are old
- Şunlar yaşlı köpek = Those are old dogs
In Duo's sentence, "şu" is an
adjective describing the dogs. (What kind of dogs? Those dogs. = "Şu köpekler.") As an adjective, "şu" cannot be pluralized, so it must remain "şu" even when it means "those" (or "bu" if it's "these dogs"). But that's okay, because we know that there are several dogs from the word "köpekler".
If, however, "şu" is a
demonstrative pronoun, (as in: "Those are dogs") we can pluralize it to become "şunlar". But in that case, we would skip the "-ler" on "köpek", because "şunlar" is doing all the work. (Those are old dogs = "Şunlar yaşlı köpek")
The suffix you're referring to is for the accusative case, which is used for definite/specific direct objects. That means it's the word in the sentence that is being acted upon (or "verbed", as some people call it).
So in the case of the apples, they were being eaten-- they are objects of the sentence, so we use accusative case. These dogs, however, aren't being eaten (thank goodness!) or seen, found, washed, or any other verb. They are the subjects of the sentence being described as old, so we don't need the suffix. (Technically without the suffix it's called nominative or absolute case.)
A last note about the Accusative case so you're not cursing me later... Just be careful because in Turkish it's only for DEFINITE direct objects--not an apple/some apples, but the/bu/şu/benim/etc apple(s). Not apples in general, only specific ones.
Hope that helps :-)