I think it might be a good thing to narrow it down for yourself, look at what you are aiming for with your game. If you want to reach the biggest range of people to play your game, the casual gaming audience might suit your needs (I'm not pointing out facts here, just so you know) or if you want to educate people on a certain topic, for instance you want to learn little children to recognize animals, you will have a totally different audience.
When you got something in mind that you want to achieve with your game, you can settle down to a more specific audience and do some research. In case for example, an educational game I mentioned before it might be of great value to figure out at what age little kids are able to identify animals, will they be able to read? Do I need to use pictures? sounds? What is interesting to them? You can go as far as to make a prototype of your game, let the children you have in mind play the game, don't tell them anything, observe, do they understand what to do? does it seem like they're having fun? are you accomplishing what you are aiming for?
There is a lot to go in depth for and there is most likely a wide variety of information on the web available when you narrow down your audience. Some general tools that might come in handy when researching are surveys or interviews. Go to your local arcade (if they still exist) and observe. Ask people why they played that game, did they like it? why not or so?
I'm sure you can come up with a lot of questions when you identify your aim. Good luck!
