Bitch where!!!!!!!
http://www.mangago.zone/read-manga/kimetsu_no_yaiba/nbt/mpn_chapter-2293001/pg-6/
Chapter 203 page 6
In a previous chapter it was said that anyone who has develops the mark on their body would use up the rest of their life force an only reach like age 25. I can't remember the exact chapter.
I forgot about the mark and its curse. I don't know why when he Muzan said a few more years, I thought he means 20-30 years. But then Yoriichi lived until he was 70 or 80 right? So it is possible that Tanjiro could live past 25 years old.
I guess we won't know until the author says something but my understanding was that yorichii was a special case. Based on that tanjiro will have like 10 years max and so will some of the others.
Chapter 170 page 8-9
http://www.mangago.zone/read-manga/kimetsu_no_yaiba/nbt/mpn_chapter-2059326/pg-8/
It was said that in exchange for power you use up life force bringing your life expectancy to around the age of 25. The author could make it so that tanjiro is also an exception. But seeing as this author kills main characters left and right I doubt it. Plus I've always felt that this manga won't have a true happy ending if that makes sense. Like I knew a lot of people weren't gonna make it and I won't be surprised if it does end up being that the ones left alive live short but happy lives.
I like how the author threw in that tanjiro will only have a few years left to live when he becomes human, as if I wouldn't notice that.
This author really be out to kill everyone. Even the people who survived in the end aren't gonna live long lives. Like wth. I love this manga but I swear all this author does is try to depress me each chapter.