It’s hard to answer this question because people of different ages are more susceptible to different types of cancer. For example in older men, protstate cancer is more common. But in the 20-39 age group, testicular cancer is most common. In the under-15’s, leukemia is the most common cancer.
As you get older, you risk of getting cancer increases, so I suppose the most vulnerable people to cancer are those in the age group 50+. They are also more vulnerable because they are not as fit and strong and so not as able to fight cancer as younger people.