"They were young."
Translation:Ils étaient jeunes.
I think it's a question of choice of the speaker. Some might say "quand ils étaient petits" and others "quand ils étaient jeunes", when talking about kids/children. Depending on the context it can mean the same thing. Obviously if "they" were teenagers, you would say "jeune", but both can be used for children. Sometimes you have to follow whatever Duo is saying in a particular lesson.