What age group does leukemia affect the most?

Increases with age. in general, the incidence of leukemia increase with age. With AML (acute myeloid leukemia), for example, the peak incidence is in 80-year olds. Acute lymphoblastic leukemia (ALL) increases with age, but also is increased in young children. Fortunately, they have a higher cure rate than do older adults.
It depends... Generally speaking, children are affected the most. Leukemia is the most common cancer in the pediatric population especially the type called b lymphoblastic leukemia formerly known as precursor b-cell acute lymphoblastic leukemia (b-all). The elderly also have an increased frequency of leukemia and other hematologic malignancies.