"When I am 18, I will..." or "When I will be 18, I will..." [duplicate]

If 'when' is used to indicate a context in the future, the 'will' becomes redundant and 'when' is followed by the present form of whatever verb.

In this case: when + subject + present simple , subject + future simple

When I know the answer, I'll call you

When he leaves for work, I'll telephone.

Of the two sentences asked about, the first is correct and the second is incorrect.

You can also say "When I turn 18, I'll take my driving test".

You don't need "will" in both clauses, only the main clause (the second one in all these cases).

English has no future tense, but can express the future in several different ways. Will (or, in speech, ’ll) + the plain form of the main verb is one of them and is typically used to make a prediction or to express a decision about the immediate a future. For this reason, it is clearly not appropriate in your example.

Another way in which English expresses the future is by using the present tense, and this is typically used to talk about future certainties. In the example, the speaker thinks that being 18 one day is a certainty, and that is why the present tense is used.