iPhone has 51% market share in Japan across all age group[1] (and even as high as 84.8% in some demographic[2]). From my 5 years of living here, I’ve never seen anyone use iMessage even once. The dominant messaging app is LINE.
[1]: https://www.idc.com/getdoc.jsp?containerId=prAP51933524
[2]: https://japantoday.com/category/tech/study-shows-iphone-the-...