If it rains, I shall stay at home. (indicative mood, because it is based on supposition)
just as "what will you do at the age of 66?"
If you were 66 years old, what would you be doing (subjunctive mood, because it is imaginary)
Think of yourself, at the age of 66, what do you think you would be doing?
(this is also correct, because I ask the question now, so it should be in the present tense, but the above "would" is used to indicate imaginary question )
Am I right ? :-D:-)