I've never killed off a character for being too old. The closest I've come was with Delvin, who was in his late 70s in game terms, but with him it was more that the character was tired, and his story was more or less played out.
Alanna's over 80, but I don't know if I'll ever be able to retire her, as such. I keep hoping she'll die in battle someday...she's far too stubborn to just fade away.
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