For me it was by watching Junjou Romantica in Youtube when I was 16. I mistook Misaki with a girl and I thought it was shoujo. So, it was a real shock for me, I didn't even know yaoi existed in general and I was super innocent even at that age. Anyways, at first I felt disgusted (I grew up in a quite homophibic family), but after that shock I became curious and kept watching. Now I only read yaoi lol.
I was around 11 or 12 when I first started watching yaoi it was something new for me, I started reading manhuas around 13 and I became a fan. It was kinda yuri at first but then yaoi reply