Are you researching the time periods and settings in which you choose to write? If you research things, there's really no excuse not to "know" how someone lived. Of course, there is a difference between living something and researching it; no matter how much you research, you're never going to be able to know exactly how something feels to someone, but you don't need to be that precise in your descriptions. If you were that precise in your descriptions of the situations you put your characters into, there wouldn't be room for anyone else in the scene; the reader wouldn't be able to put themselves there with your characters.
Holly's right when she says that most writers draw from their personal experiences in some way. It's just a matter of how deeply you draw from your life when you write. There's another quote (and I'm paraphrasing because I don't know who said it or exactly what they said) that says something to the form of "you have 50% (or more) of the material you need to write by the age of thirteen." To write, you need to live life. You need to live life from all different perspectives. Sometimes, the only way you can do that is through researching a topic (especially if you're writing something historical) until you can write about it as though you lived it.
On a personal note, I'm seventeen. I'm a girl. I've never had a kid, don't plan to for several more years, and haven't held a job outside of being a high school student. Does that mean that I can only write about seventeen year old girls who go to a private school like mine? No. I think that's boring, so I write about soldiers, mobsters, murderers, both in modern and historical contexts. The one thing people have never told me is that they can't relate to my characters. I've never written about a seventeen year old girl struggling to get her AP US history notes done on time, even though that's the life I live.
I could write about myself, yeah, and maybe I will write about that high school girl someday. Just not now, when I'm living it.
Writing's all about being flexible. =D
edit: on second thought, writing is also about finding the humanity in someone. If you can find the humanity in someone, you've found your relatibility.