Translation:My older sister is twenty four years old.
That is a good question! I am not certain, but I believe that 岁 indicates the verb along with the age. In my other learning programs, I have never heard 他 是 二十 岁。 That, of course, does not mean it is correct! So, we could use a native speaker or someone further along to answer this... :)
I wrote "my older sister is 24 years of age", instead of "my older sister is 24 years old". It means exactly the same thing in English. Needless to say I got marked wrong, not for the Chinese being wrong, but by the limited choice of English responses.
I'm a mother tongue English speaker.