When you're working on a game, you hear this question a lot. If you've worked in the entertainment industry, or heard it talked about, you know the kind of answer people are expecting. The format is _Gender(s)_ _Age Range_
The more I've been forced to answer this question, the less meaningful I find this kind of answer. The question is, without a doubt, still a meaningful one. Who do you think would most enjoy this game? No game is for everyone, and, practically speaking, marketers need to know where to direct their efforts and their dollars. There are also certain kinds of products which are perfectly justified in using gender and age. Acne cream, for example, will mostly be sold to teens, even though lots of other people have acne. On the other hand, I think there are ways of dividing the population which might be more valid (i.e. more predictive) for interactive entertainment.
Sometimes the simplest conventional thing to do is to point at a genre. That is to say, the best predictor of your enjoyment of the next pixel platformer is your enjoyment of the last pixel platformer (or, perhaps, whether you know what a pixel platformer is). This is a rudimentary form of segmenting by play-style, which is a strategy with a lot of depth. You might consider defining your audience in opposition to other games' audiences, or court people who liked a certain game except for one particular thing.
In my case, I'm working on an educational game, which is a concept that comes with a lot of baggage. People assume that it's for kids, and probably young kids. And it is. But it's for a lot of other people, too. My game isn't for everybody, but it's not about gender or age.
Codemancer is a game about learning programming, but it's not a puzzle game, or a sandbox environment. It's for people who are interested in story-driven interactive experiences. It might be for people who are somewhat interested in learning to program, but not totally obsessed with learning to program (the obsessed don't need my game or any game).
I hope that my game is for people, especially women, who wouldn't normally be drawn to the "hacker" archetype. It's for people who can't stick with open-ended learn-to-code creativity tools like Scratch* and Kodu*, or who might be ill-suited to online tutorials, like the ones at Codecademy* or Code School*. Maybe they don't have access to in-person classes, like the ones at Starter League*, or are too embarrassed to expose their ignorance to a class full of strangers.
So now when people ask me what my game's demographic is, I say "what's Harry Potter's demographic?"
Then they say "But really..." and I say "Fine. Males and Females 8-11 years old." I've got to pick my battles.
*These are all cool things that you should check out.