"Mit zwölf Jahren ist ein Hund alt."
What does 'with 12 years' mean in English? Wouldn't it be more correct, in English, to say 'at 12 years of age'?
With stuff like this I've given up trying to get an exact translation. As long as you understand what it means in German and can convince the website that you do then it should be ok.
OK, why did they tell me "With twelve years is a dog old" is wrong? I understand it isn't a pretty sentence, but it gets the meaning across...
Because JungleFritz some of Duolingos' English is bad, very bad on occasions. I agree with jpentland, just convince the website, write the translation down if its a really odd one.