Answer
The onset of leukemia is earlier than that of other cancers because the mutations necessary for it are significantly fewer.
Work Step by Step
Leukemia is a mutation that causes an excess of white blood cells, which are notoriously invasive because they are meant to fight off ailments and infections. They can travel wherever they choose in the body. This means that if they are in excess, there is no need for any further mutations because the white blood cells can already travel wherever they want in the body. Another mutation would be required if they weren't already invasive and capable of traveling so easily, but as there are so many, it is not necessary.
