Vincent Teo“ Can I say,
(a) Susan and Suzana are good neighbours. They have / had been good friends since they were ten years old.
(b) They have / had been a good friend since ten years old.
”
(b) Philip wrote: Either could be correct, depending on what else occurs in the sentence.
However, it should be "since ten years of age" or as you have it in (a).
(a) Susan and Suzana are good neighbours. They have been good friends since they were ten years old. ( Isn't this correct? Why should it be 'since ten years of age'? )
Many thanks.