For me, it was when I got my first laptop at 13. Puberty hitting like a truck at the same time, it was simply impossible to stay away with the internet at my fingertips.
First porn I saw was printed (magazines, etc.), it was way before I had my first PC, and internet came much later. I never really wanted to specifically watch porn, but my natural urge for sex eventually drove me into that. Those first erotic magazines I saw were just looking curious for me because they depicted something I never saw IRL. It was forbidden, I had nobody to talk to about it, I didn't even know how to masturbate, but I wanted to know more. Later, when I heard from some of my friends at school how to masturbate, I started using porn for its real purpose.
Because we can. By that I mean it’s so readily available to us. I think initially it’s nomal human curiosity for most. But why do some people like it more and get addicted? The same reason people try smoking and don’t get addicted while others do or how many can have just one glass of wine.