I think I was about 13/14. I found a site with streams of JPop and JRock songs and I started listening to some songs by Ayumi Hamasaki and Hitomi Yaida (because they had been recommended to me). Then I found songs by Youjeen (I love that woman) and when I looked for more rock-y stuff I started listening to L'arc~en~ciel, Luna Sea, LIV, Malice Mizer,, Dir en grey, X Japan (I think Kurenai was one of the first JRock songs I heard), Buck Tick, Penicillin, etc. And it sort of grew from there.
Since MM were one of the first bands JRock bands I heard, I guess I discovered Visual Kei around the same time, lol.
And I know for a fact that Youjeen was one of the first J-music artists I heard and she is still one of my favourites, despite her not releasing anything anymore.