"The brother of Sebastian is eight years old."
Translation:Fratele lui Sebastian are opt ani.
That is not the case at all, as far as my knowledge goes. "are" is the third-person singular form of the verb "a avea" - "to have". So it is used when saying "he/she/it has". The small word "a", on the other hand, has a number of uses. It is generally an auxiliary verb or word with many uses.
For example, you'd use it to form feminine ordinal numbers: "a treia femeie" - "the third woman"
It is used to form feminine abolute possessive pronouns: "pisica asta este a mea" - "this cat is mine"
And it is also very often used to indicate the dative/genitive case.
However, I've never encountered "are" to be shortened to "a", I don't know if there is some dialect that does that. There are some similar pairs like "este" and "e" which are the same thing and interchangeable.
I haven't tried this answer since I got it in a word bank problem, but at the end, would "are opt de ani" work? If not, why? I saw that format in another sentence (something like "The grandparents of these children are ninety years old" where the end was translated as "au nouazeci de ani").