Most people with Alzheimer’s are older and would probably trade having their cognitive abilities back even with increased risk of cancer

Depends entirely on the stage of the disease and the aggressiveness of the cancer! Getting an aggressive brain cancer when you had early stage Alzheimer’s [1] would be tragic. The tradeoff would be years of life.

For the record, I have no idea what the actual risk tradeoff is, but the point of regulation is that nobody does. You can’t have informed consent when you can’t be informed.

[1] Aside: Alzheimer’s is relatively early stage, as dementias go. It’s frequently diagnosed by onset in younger people.