"They are old."
Translation:Oni są starzy.
Przymiotniki kończące się na -y tworzą formę mianownika rodzaju męskoosobowego ze zmianami: adjectives ending with -y form Nominative masculine-personal with changes:
r → rz (stary → starzy, mądry → mądrzy)
s → ś (żałosny → żałośni)
końcówka -ty → -ci (popielaty → popielaci)
końcówka -ły → -li (biały → biali)
końcówka -ny → -ni (wolny → wolni, czytany → czytani)
The rest of the word choices were for "są" and "starzy", which makes sense that it was flagging "One" as wrong, but then "Oni" should have been in the word bank. I flagged the question but there was not an option to explain the problem, so I mostly posted the problem here to bring it to someone's attention.
I was using a web browser, so ultimately I changed to the keyboard option to type in the correct answer, but if I were using the app this would not have been an option and I would have been stuck in a loop, unable to answer the question correctly.
I have to say that this is really weird... if anything like that happens again, could you provide a screenshot? Without a screenshot we are not really able to investigate it further or bring it to the staff...
On a separate note, "Są starzy" itself would be accepted. But it's not a starred option, so it's not what the exercise should expect from you, it's just an acceptable answer... all the words from the main answer have to be in the word bank and if they're not, that looks like a serious bug.
The pronoun and the adjective don't match each other. Basically you have two options:
"they" include at least one male person, so it's 'masculine personal plural', aka 'virile'. The right pronoun is "oni" and the right form of the adjective is "starzy".
"they" do not include any male people, so it's 'not masculine-personal plural' aka 'non-virile'. The right pronoun is "one" and the right form of the adjective is "stare".
In most Duolingo sentences there's not enough context to say which option is meant, which makes both options equally correct.