There are so many different types of cancer in addition to individual people’s tolerance level for the side effects of chemo and radiation particularly that it’s impossible to generalize to as simplistic a degree as presented in the question.
It’s also dependent upon whether the cancer is classified as a stage one (very early diagnosis) or stage four (very advanced and difficult to treat).
Also cancers differ in their rate of growth and destruction. There are many types of Prostate cancer which are so slow growing that it’s likely that an elderly man may well die of plain old age before Prostate cancer.
But there are other far more aggressive and hard to discover types of cancer (such as Pancreatic cancer) which typically leave a person with a year or less to live regardless of the treatment. So, in that case, the person might well opt to just enjoy whatever time he has left rather than suffering through chemo.
So, that’s just two brief examples of why there is no simplistic, one size fits all answer to the question you pose.
PS: how about a link to this “study” you reference? I have a hunch it’s unlikely to be from a reputable source (like Mayo Clinic, for example).