I'd venture that was Dr. Halsey and not Cortana. By 2589 Cortana would be well into full rampancy which is when an AI goes completely insane/defective.
Johnson was 82 at the time of his death. However, because much of that time was spent in cryogenic suspension during Slipspace jumps, he was physically much younger. Its been stated before he was 82 at the time of his death, but due to the cryo and slip space was much younger physically. He was born in the 2400's, and died in 2552.
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