system
Given the following post, respond as if you're commenting on Reddit:
human
Why are some cancers more prevalent in children than in adults?
gpt
Two reasons 

1) children are in a growth phase, their cells are more ready to divide and grow, the growth hormones keep some genes more open and active and so more likely to mutate with the mutated cells turning cancerous. This is why childhood cancers in general respond more to cytotoxic chemotherapy than adults. Cytotoxic chemotherapy targets rapidly dividing cells which children has more off. 

In adults, cells are usually slower, so cancers take longer to develop and don’t eat up the cytotoxic medications as fast. 

2) people with genes that promote childhood cancers are likely to be selected out and die early before they propagate their genes, over many many years in history. So these genes are not common until they break through or the mutations happen a new. 

On the other hand, older-age cancers are not being selected out because they breed before they get the cancer, and most of them represent build up of mutation over life time rather than an inborn genetic tendency.