I guess I can trace the start of my RPing "career" to messing around on AIM with my sister and a friend playing original character Sailor Senshi. They were Zodiac Senshi, soooo creative. XD We created characters for ourselves, but we didn't put much thought into building a continuous story at all. We kind of just threw our characters into a situation and ran with it. I was about middle school age back then. I used to draw my character all the time, ha. My drawing skills are none to be envied even till now.
I started formally RPing on special interest messageboards (Pokemon, Digimon, Inheritance Cycle) when I was about 16 -- apparently I started pretty late. Since RPing wasn't the focus of these boards, the games tended to be small and died out within half a year at most. These were all fandom-based original character games, and I won't hesitate to say most of them were HUEG Mary Sues. But I was still a newb then, and I was just finding my way into the medium. Even though I can see their flaws now, these characters still have a special place in my heart and memory.
I didn't get into journal-based RPing until 2006. My preferred journal site is LiveJournal, but I dabble here and there on GJ and here. Mostly I play fandom characters these days, but I like to keep an eye out for an interesting original character game premise. I check out IJ mainly for the OC games while sticking to LJ for the fandom ones. I do some storybook logs over AIM and GoogleDocs for the journal games, but my recent games are based more on threading so I haven't had a need to use those platforms to RP much. I'm dipping my toe into doing PSLs, but because I'm a very shy person I tend to clam up during one-on-one lines. I also take longer to tag back than most people do, so I'd worry about boring the other person.
The LJ RPG scene is... let's say a mass sociology experiment that has yet to be properly studied or documented. It can be a really frightening place sometimes. You find RPing is almost like high school with the cliques, backstabbing, and gossip. It's like RPing is divided into two parts: the creative writing part, and the out of character relations part. I like to believe I'm okay to decent at the former; the latter can use a LOT of work. I generally don't involve myself behind the scenes of these games, so I've yet to be personally attacked for my IC or OOC behavior, but there are many a story where people have been hurt because of the dark side of LJ RP. Still, I enjoy it when I'm on a stride, and despite my social limitations I imagine I'll still be RPing in the future.