Is it legal for a bank to sell a person with dementia a Life Insurance Annuity?

Selling anything at all to a person with dementia - is the taking of advantage of a person who may not at the time be capable of making a judgement of any kind. I would view as SUSPECT - any sort of sale - of any value - if it can be proven that at the time the seller KNEW the person to whom they were selling, was not in full possession of their faculties -