Why when people die, we add them a year

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Some Croatian singer died today. I think he had a good life; he died at the age of 97 but the news are saying he died at the age of 98 cause turning 97, he stepped into 98 but, for me, he didn't. Why adding one more year. I always thought numbers are valid but not sure. also, for many things here, maybe at your place too, for example, if it says read until page 6, sometimes that page is included and sometimes not. Yes, these years with dying are messing with me. I remember my grandma died when she was 89 but on her tombstone, it says 90. she wasn't 90. She died 2 days before her birthday. Hm...