Slash was, apparently, though I had no idea at the time that that was what it was, a relatively early interest for me. I remember thinking up 'slash' stories (that were never written) when I was in my early teens. In fact, I had a dream (real night time dream) about two 'brothers' in a tv series and at the time, I didn't even have any idea of what exactly those two had been up to, but it seemed very interesting to me. That continued for a while, until I learned more about gay relations etc. I didn't know anyone who wrote either slash stories or for that matter fan fiction. Maybe it's because I'm from a rather 'provincial' country or that I'm pretty old by now (I hope my readers are now saying no, you're not!) but I remember people talking about gay relations as being somehow wrong or sick. Another example: I have a second cousin, who's quite nice, but maybe a bit simple. Some pop stars came up in conversation (this guy loves pop music) and I mentioned that they were gay, and he said: no way, they seem so well behaved (as in well brought up or civilized, I guess). I didn't get his meaning - well, in a way I did, but it seemed so totally irrelevant. So clearly in his family (not very surprising, because you really should know a few things about his mother), it's the way they think about gay people.
From about the same time I began to get my slash ideas, I tried to write (extremely bad) romantic stories about straight couples, but they tended to bore me in the end. Traditional couples don't interest me. I always like something different, like f/m or the woman being older etc. Not that I even like 'pure' romances or will write them. It's also extremely difficult to write a good straight pairing in fan fiction, because precious few female characters are any good, to be brutally honest. That leaves you to invent an original character, which isn't very popular, but I have done it in the past (no Mary Sue, more like an anonymous, independent watcher from outside the group the writing is about) or to improve the female character so that I like her, which may not be very popular either, because everyone has a different view of what is a 'good' female character.
In original fiction, I've been told it's not really popular to write about female characters (at least in children's books) because apparently, girls can be expected to read all books, whereas boys can only be expected to read about other boys. Relatively recently (a year or two ago), I remember reading about how critics were lamenting the loss of the most popular children's books series (due to age I suppose - the writers are no longer up to making up new stories), because they were more or less the last ones who write about boys. Now, I'm not that picky - I also never pay attention to what colour or ethnic background the main character has. All I care about is whether it's a 'good' character (as in one I like).
In fact, I don't really worry too much about the age of the main character (though I'm getting more than a little tired of what is in Swedish referred to as the 'middle-aged male mystique', meaning how you get to dwell in detail on how these men don't brush their teeth, how they prefer to have a drink instead of eating a proper meal, how they don't bother to shower, don't obey their superiors and yet, despite all these drawbacks, are irresistible to significantly younger females.