What is the oldest age that you can get childhood leukemia?

Pretty old for kids. The most common leukemias that occur in kids also occur in adults. It is possible to have leukemia as an young adult and be treated according to pediatric protocols, which generally are more intensive. The protocols for childhood leukemia allow for treatment of "kids" up to 30 years of age.
Complicated. The most common "childhood" leukemia is acute lymphoblastic leukemia (all). All was uniformly fatal & now the survival is >80%. It is one of the great success stories of oncology. Teens & tweens have been found to benefit from "pediatric" chemo regimens more than "adult" regimens. A recent study www.Nature.Com/leu/journal/v25/n11/full/leu2011141a.Html suggested a benefit (w/ allo sct) up to 40yo.