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The normal elderly person can live for a few weeks without drinking or eating as long as he has the adequate nerggy resouces in his body and he is not severly degydrated to land up in shock and death.
Once the energy resouce is exhausted then the person dies.
Though there are some spiritual people in the world who have spiritual power and are claimed to live without eating and drinking for months to years.
The more study and information is needed to prove of such conception.
Please visit here for more information:
Article bbc news
Let me know if you have any doubt.