be/become

be is static.  It indicates a given state or condition.
become is dynamic.  It indicates attaining a given state though a change from some other state.

The weather was warm.  No change is indicated.
The weather became warm.  It was cooler before.  It changed to being warm.

CJ

They are both correct.  Which do you want to emphasize?  That your age will be 18?  Or that you will become 18 by changing from 17 to 18?

This is not a question of correctness, but a question of what is more idiomatic, that is, a question of how people usually say it.

Most people usually say I will be 18 next year, emphasizing what the age will be.  (The change from the previous age is not so important for most people.)  So, to match your usage to that of most English speakers, use be, not become, when talking about age.

CJ

Hi,

First, consider this. Do you know the difference between Mary cooks dinner and Mary cooked dinner ?

Best wishes, Clive

Dear Clive:

Thank you for immediate reply. Mary cooks dinner indicate that Mary cooks dinner all the time. Mary cooked dinner tell us it was in a past time when Mary cooked dinner.

With this understanding in mind, I would never say: John becomes ill. Because, too me, it is always in a past time John started to feel ill. So I would say only John became ill is logical.

However, we can say: John is ill. Or, John was ill.

Best regards