The elder stage doesn't have a definite ending so it just tells you how old they are in total. They live at least 10 days (in your setting) and after that, they can die any day. It's just not definite when this is. It depends on your live stlye and traits.
What the game does can be compared with rolling a dice each day after they reached the 10 days in elder stage. You have a certain probablity with which the Sim dies which gets smaller each day.
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