According to the Leukemia Society of America, "[i]t is suspected that many cases of childhood leukemia may result from possible exposures to either an infectious or toxic agent during fetal development or early childhood. Acute lymphocytic leukemia occurs most often in the first decade of life but increases in frequency again in older individuals."
[Source: http://www.leukemia.org/docs/pub_media/all/fc_causes.html]