Is there an age group that leukemia affects the most?

Not really. We see different types such as acute lymphocytic leukemia in chidren and chronic myelogenous leukemia in those over 40 but we do see these diseases across the age spectrum. Certainly the very youg and the very old have a higher incidence rate.
Older... For all, children rarely get leukemia but this is the type they usually get. For adults, AML is the most common acute type and its incidence increases with age. The chronic leukemias also increase risk with age.