I agree^^;;;
I think Inuyasha's ages can't used human ages to count^^;;;
If you really have to count his actual ages in human well...Inu-Chan's father was passed away about 200 years ago while Inu-Chan was bornt.Imagine Inu-Chan at least have 200 years old than divided by 15 he would be about 13 years to grow up for a year^^;;; (Which remains me that Takahashi-san had mention that don't know how to count Inuyasha's ages,but he's as same ages as Kagome though^^ )
So maybe because of that,Inu-Chan's mather passed away^^;;;
Anyway,just leave it as mystery.I think that's what the artist think^^;;;