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What is the relation between age and cancer development probability? I have heard that cancer is rather rare at younger age groups.

Assuming it increases with age, is there a peak after which the probability declines?

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The National Cancer Institute states

Advancing age is the most important risk factor for cancer overall, and for many individual cancer types. According to the most recent statistical data from NCI’s Surveillance, Epidemiology, and End Results program, the median age of a cancer diagnosis is 66 years. This means that half of cancer cases occur in people below this age and half in people above this age. One-quarter of new cancer cases are diagnosed in people aged 65 to 74.

Lung cancer, breast cancer, and prostate cancer all have median ages of diagnosis between 61 and 70 years. That said, the NCI does note that some cancers occur more often in younger age groups, citing bone cancers (where the most at-risk age group is adolescents under 20) and leukemias.

The NCI also provides a bar graph showing the percentage of new cancers in various age groups, broken down mostly in ten-year increments:

This would seem to indicate that your cancer risk goes down after about the age of 70. However, this is clearly misleading, because fewer and fewer people survive to reach these higher age groups. Thus, with fewer people in that group alive, there is going to be a significant drop in the percentage of cancers that occur in that group.

More accurate tables exist, which show the chance of developing cancer within a certain amount of time at a given age (data from 2010-2012). Table 2.12 in the linked document shows that the chances of developing cancer within 10, 20, or 30 years increases with age (although long-term data is not available for the higher age groups, as most people at the age of 80 do not live for 20 or 30 more years.

An interesting effect, though, is that the chance of ever developing cancer does change, increasing by small - negligible, even - amounts from the age of 0-30, and decreasing by the same small amounts from there, until there is a sharp drop-off at the age of 60. The table shows that if, for example, you're 70 years old, then you're less likely to get cancer during the rest of your life than someone 40 years old. The reason for this, of course, is more morbid: other diseases that are more common or more deadly in old age will set in at older ages. Less time to live implies less time to develop cancer.

To make this less confusing, at younger ages, your risk of developing cancer within a small amount of time - your instantaneous risk - steadily increases. At ages past 30, however, your risk of developing cancer throughout the rest of your life steadily decreases.

I would assume that your question is better answered by the first statistic, rather than the second.

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    I'm surprised the NCI didn't make that graph more useful by normalizing it for "percentage of population in that age bracket". Sounds like something that wouldn't be too complicated if you had the raw data anyway. – YviDe Jan 16 '16 at 19:14
  • The most interesting part is that about 90% of cancers appear after age 45. Which was why I actually asked the question; I wanted visitors to see that below that age their cancer risks are much lower, so that they are not that worried that they ll get cancer when they are young. (maybe emphasizing it would be helpful to them). – Fermi paradox Jan 17 '16 at 11:47
  • An interest[ing] effect, [...] 0-30 and steadily decreasing from there. - I am not so sure about this. If you are referring to the data at page 13, the difference in risk looks negligible before age 60. However, it does become non negligible after age 60 (probably for the reason you mention). – Fermi paradox Jan 17 '16 at 12:59
  • @Fermiparadox Hm, the link is bringing me to the first page. Regarding your last comment: I perhaps misrepresented the trend. I was trying to show that there is a peak at age 30. You're right, though, that the differences are pretty much negligible in that area. – HDE 226868 Jan 17 '16 at 15:12

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