According to a study from USC School of Gerontology published in the March 2014 issue of Cell Metabolism, a moderately high protein diet in people over 65 is good for you, but in middle age leads to a two fold increase in oveall mortality and a four fold increase in getting cancer. Protein intake controls growth of IGF-1, which has been linked to developing cancer.
On the other hand as levels of IGF-1 drop, as they do over the age of 65, it leads to muscle weakness and frailty. So in younger people, high levels of IGF-1 lead to more cancer, but in the elderly it helps prevent muscle weakness, frailty, and injuries from falls. For every 10 ng/ml increase in IGF-1, for those people on a high protein diet there was a 9% increased risk of dying from cancer when compared to those people on a low protein diet. Researchers also found that just changing from a moderate to a low level protein diet reduced the risk of cancer by 21%.