Translation:They used to start school at the age of six.
They started school "at six years old "....... is JUST as correct in proper English as "at the age of six" somebody needs to get a grip here and learn English!
Duolingo marked my translation into English wrong, trying to second-guess how it wanted 6 years old to be expressed in this exercise. Instead, it suggested "They were starting school 6", which is gibberish. I'm starting to worry that equivalently, Duolingo's teaching me gibberish Romanian.
Agreed, "at six years old" is perfectly normal. It's frustrating being flagged as incorrect for invalid reasons. The exercise is to learn the imperfect tense, not second guess how phrases should expressed.