'When I will ask this question, it will mean that I will have become truly old' is the exact translation you want me to give, but it's a sentence no English speaker would say. It becomes a matter of memorizing the answer DuoLingo wants in order to get through the lesson. I'm not whining; you provide a great service and I thank you, but more attention to English translation, especially in these later lessons, would be appreciated.
Maybe 'If I were to ask this question, it would mean I have become truly old' or just 'I would have to be really old to ask such a question.' It's hard for me to say, because Duo is trying to teach me a particular grammatical lesson. Language usage doesn't translate perfectly anyway. Probably in Italian people do say things exactly that way, but they don't in English. Thanks for asking. One of the bonuses of language lessons is you start to think about your own language.