Question

By the time you're my age, you ___ your mind?

A: will probably change
B: are probably changing
C: would probably change
D: will probably have changed

Answer 1

The correct option is -

D) will probably have changed your mind.

The sentence becomes :

By the time you are my age, you will probably have changed your mind

Answer 2

By the time you’re my age, you will probably have changed your mind?

Answer: Option D

Explanation:

The tense that has been used here is the future perfect tense. Future perfect tense implies of something that is bound to happen in the distant future. Have or has is used here depending on the verb. For example: He will have to leave by then.  

In this sentence, future perfect tense is used as it is in agreement with the subject. The speaker tries to convey that by the time the listener reaches his age he will by then have changed his outlook. Hence this form is used.