The phrase comes from cricket, where a good innings means a player scored well. At 95 years old with a lot accomplished, he certainly had a good innings.
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Extended Metaphor
It's often used metaphorically to describe life or career duration, not just death or retirement. After 50 years of teaching, Mrs. Brown had a good innings.
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Cultural Usage
Mostly used in the UK and Australia, might be less understood in other English-speaking countries. He had a good innings, said the British man about his American friend.