For some reason it is just something that happens in our bodies that we have no control over. We can't sit and wonder how we got it. There will never be an answer. We just have it. There is no certain age when you get it - not like the chicken pox where most people get them when they are very young.
People sometimes think though that only older people get cancer.
There is no way of saying who will get cancer and who will not get cancer. There are some forms of cancer in which certain things that you do can increase your chances at getting cancer rather than decreasing the odds, like if you smoke, you are more likely to get lung cancer that someone who doesn't smoke. But for most cases, there is no way to know who will or will not get cancer. And not everyone gets cancer. But you know what, there are more people out there than you think that have or have had the disease.