Great question! You are right that cancer is due to rapid division of cells, that neurons don't usually divide after the brain forms, and that the brain has more cell types than just neurons. That's pretty impressive for someone your age. Brain cancer can come from cells other than neurons. For example, glial cells support neurons and help them work. They can become cancer cells. Cells that form linings, coverings, or glands in the brain can all form tumors, which can be cancerous.

Some cancers in the brain come from cells in other places in the body that travel to the brain.

Replacing lost or damaged neurons would seem like a good thing. Why do you think they don't usually divide?

You seem to have a promising future in a field like cell biology.

This is a very interesting question. While neurons in an adult mammal do not divide, there are many kinds of supporting cells in the brain, such as glial cells, blood vessel cells, and meningeal cells. Thus, brain tumors in adults could develop from these types of cells. However, it should further be noted that many tumors found in the brains of adults are actually the result of another types of cancers that have metastasized throughout the body, including the brain. As another note, while neuron cells do not actively divide, there is evidence that adult neurogenesis (neuronal cell production) is possible in some regions of the brain!