

I just looked it up on the wiki, and the closest thing I can find is the 23-day rule, which isn’t mentioned in the anime, and only adds the condition that if you specify a specific time within 23 days and specific disease that wouldn’t be fatal in the timeframe you specified, it defaults to a heart attack. If you specify a disease that takes over 23 days to develop and no specific time, then the disease will progress starting when you write at its natural speed. Old age as a cause with no date seems to fit the latter category.


https://deathnote.fandom.com/wiki/Rules_of_the_Death_Note/Manga_Chapter_Rules doesn’t mention anything about a 30 day limit, and does say that any human is considered capable of suicide. I watched it closer to my birth than to now, so I’m just going off vague memories of what I thought when I watched it and the wiki, which might not be perfect.