If the denizens of Medieval Europe ate moldy bread on a regular basis, they wouldn't have had the same problem with the Black Death as they did. That's (usually) penicillin - and would have cured a lot of ailments during that time, including the Bubonic Plague. It also hasn't been found to be cancer causing. If you're referring to the particular subset of Black Mold that is thought to be cancer causing - that's in our own food supply as well. Some estimates point to 50% of the food we eat having some amounts of the stuff.
On the topic of lead - that's less proven than high levels of electromagnetic radiation to cause cancer. It sure does cause a lot of other problems, however.
As for living under power lines - or working in close proximity to high voltage fields, there have been a number of studies that have shown pretty clear links. Linear increases in cancer risk per the amount of exposure. The original study, done back in the 70s, showed a link between power lines and child leukemia. Of course, more recent studies say "inconclusive" because of too small a sample set. That's because everyone took the damn hint and stopped living under power lines. It even became a running joke - "What, were you raised under power lines?!".
Quite frankly my comment was in relation to that - to be mildly amusing, and to point out that our modern world has a few more cancer risks for children and young adults than the medieval or ancient worlds might have had. It does. EMF, in general, does appear to come with some cancer risk. A study this year came out showing that cell phones cause tumors in mice (and there have been other studies to show increased cancer risk in humans). Workers in high voltage areas seem to have cancer more than the general populace - even less common cancers like male breast cancer. Workers at chemical plants are at greater risk, because many of these chemicals are carcinogenic. People who smoke, or chew tobacco, are at greater risk.
But the prime cancer risk, as I pointed out in an earlier post, is none of that. It's age. Seeing as we're more likely to avoid disease and war, and get into the 70s, we're more likely to die of heart disease and cancer. 89% of cancer cases in the West are from people over the age of 55, and the risk grows with each year. This was what I was pointing out. Children in game, if we are to go by some semblance of realism, shouldn't be dying of cancer or heart disease at the same rate as adults. If you go that route, you might as implement Alzheimers disease, and give the same occurrence rate to 2 year olds as to 70 year olds.