Why does cancer tend to be more common in older people?

It's a very good question and a very astute observation. It really has to do with the fact that many different genes need to be altered by their contact with whatever it is in the environment that causes cancer in the first place. It's seldom that any single gene defect is involved in cancer. People think that it takes at least six to eight genes, but it is probably many more. They must all be altered together in the same cell before you get the change in behaviour that we would call cancer. The chance of getting all these changes at the same time in one cell is actually very low, but it increases with age.