Well, I’ve managed to come back to this at last.
I started reading Manga quite early. Maybe at first I was just watching the pictures, but I think I also started actually reading them barely after I learned how to read Hiragana, one of three Japanese characters. If you have ever tried learning Japanese, or have heard about it, you may already know how difficult Kanji are. In most of the Japanese Manga, every Kanji is with Hiragana or Katakana on it. So, basically, if kids know Hiragana and Katakana, they can start reading Manga.
In Japan, there are lots of picture books and magazines for kids under school-age. I remember when my father bought me a magazine for kindergarteners. Those magazines carry lots of Manga too.
And around the same time, or maybe slightly earlier than that, I also started reading Manga for much older girls. One of my young aunt often gave me comic magazines for teenage girls. Back then, almost every girl were crazy at the so-called Shojo-Manga, comic book for girls. My girl cousins were into it, too. One of them were even aspiring to be a Manga artist!
Many Manga also have literary grace. And I have to admit I learned Japanese through Manga even as a Japanese. One of my earliest heroines was Marie Antoinette from “Berusaiyu no Bara“, The Rose of Versailles, by Riyoko Ikeda. I , merely a child back then, had been so influenced by her!