Final answer:
Chronic leukemia, including chronic lymphocytic leukemia (CLL) and chronic myeloid leukemia (CML), is more common in adults over the age of 55, with CLL being particularly prevalent in this age group.
Step-by-step explanation:
The type of leukemia that occurs mostly in adults over the age of 55 is typically chronic leukemia. Unlike acute leukemia, which involves an overproduction of young, immature leukocytes, chronic leukemia is characterized by the accumulation of mature leukocytes that fail to die. This can include chronic lymphocytic leukemia (CLL) or chronic myeloid leukemia (CML), with CLL being more common in the older adult population. Both types of leukemia cause an increase in abnormal white blood cells, which can lead to symptoms such as excessive bleeding and bruising, fatigue, fever, and an increased risk of infections.