In later life we expect to perhaps become forgetful, but none of us believe that we could lose our mental capacity, because those sorts of things happen to other people, not us. After all " I'm only 50, I'm only 60, I'm only 70" etc. Mental decline may occur slowly over time or by a sudden event such as a stroke
If you have invested funds with a bank or investment house you may well have given them authority to buy and sell shares or funds, much in the same way an old fashioned stockbroker may have acted for you.
I remember back in my days in financial services we had a rubber stamp in the office with a red ink pad that said "ACT NOW!" and where necessary we used to stamp it on client letters.
The recent stockmarket volatility after the referendum has focused the importance of active management of investments and pension funds to maximise returns and prevent losses.
Many advisers ask this question, and the truth is that planning is severely restricted but sometimes possible.