Translation:My cat is nine years old.
Because in Chinese they don't use "is/are" as often as we do in English, it is "built-in" or assumed in some cases such as this instance. Because of lessons like this one and (它六岁。(Tā liù suì. ) It is 6 years old. ) for example, we are learning on a case by case basis, when we should use "is/are" and when we should not. See a comment in https://forum.duolingo.com/comment/25213338 by mei.ren.yu - I will quote here: "You don't use the verb 是 here (ie. referring to lesson: 它六岁 It is six years old.). This is confusing when you go from English to Chinese, because we use the "to be" verb for so many things that Chinese doesn't. For example, the question "How old are you?" is 你几岁了 (Nĭ jí sùi le), so you should answer the question by swapping out the question words for the answer bits. 我二十九岁 means "I'm(我) 29 (二十九) years old (岁)."
Always remember that because English and Chinese come from such drastically different roots, that a lot our the language conventions from English will absolutely NOT carry over. Be especially careful with your use of 是.
Hope that helped!" Thank you mei.ren.yu