During my degree in physics I had to take a course in radiological protection; while not in any way an expert in these matters, I do recall something very important - pilots do receive higher doses of radiation than the general public, due to cosmic radiation at high altitudes. The higher the altitude, the higher the dose of radiation. However, the yearly doses that pilots receive are well within the safety limits of occupational exposure. Only Concorde pilots (remember at 60,000 ft) flying London-New York return every day would exceed the recommended limits in a year, and no Concorde pilot ever flew every day for a year. (I believe it was in fact my lecturer at the time in Trinity College Dublin, Prof. I M McAuley who did this particular work on radiation exposure in pilots).
Where doctors and dentists are concerned they are exposed to x-rays every day. Long term low-level exposure under those circumstances is a different matter to what pilots receive, due to the nature of the radiation (x-rays vs cosmic rays) and the frequency of the exposure. Heavy lead lined aprons for health workers involved with x-rays is a common sense precaution and not related to radiation exposure for pilots.
For most people working in the airline industry, the additional cancer risks due to occupational exposure are small enough that other factors are much more important. Smoking, for example, makes occupational radiation exposure look very meek indeed. Other environmental factors, radon gas exposure in our homes coming up through the bed rock, is a much greater risk in many parts of many countries, Cornwall already mentioned.
To answer the original questions...
1.Has it ever been proved that radiation is higher at jet altitudes (FL300 and up)?
Yes.
2.If so, is this radiation strong enough to affect the health of people over time?
Not a yes or no answer. The doses received are well within occupational exposure limits. There may be an effect but is is comparable to other environmental health risks and much smaller than some.
3.Is there any study that suggests cancer is more common among airline crews?
I'm sure there are but it would be very difficult to conclusively link any increase in cancer to occupation radiation exposure. For example - do cabin crews smoke more than people in other occupations? Is there a statistically significant link to the number of hours worked and the increased risk of cancer?
4.Do any airlines limit the number of flying hours for crew in regards to radiation?
Not that I am aware of. I am also not aware of any need to.