After watching our father die from advanced dementia, my brother established instructions that no one is to feed him. If he can't bring a spoon to his mouth unassisted, he would like to stop eating. He works at a hospital, so I believe that he understands how to do this in a legally binding way.
This tempts me to leave similar instructions if I can't cook for myself.
Such advance directives aren't completely legally binding, and rely on family and caregivers respecting his wishes. If your brother is mentally incapacitated and his next-of-kin asks a healthcare facility to put in a feeding tube then they'll probably do it.
My father's last intelligible words were "I want a pill." My mother understood this request, but we could not act on it.
A year later he stopped swallowing. My mother polled the family, and the consensus was to try intravenous fluids once, to see if this was temporary. With the fluids it took him a week to die. See how hard it is to even follow someone's last words?
Everyone, regardless of current age or ability, should absolutely have advanced directives in place outlining what they want done (or not done) to prolong their life.
Talk to your doctor about filling out the appropriate paperwork.
Even if you want everything humanly possible done, it's best to document it, and keep that document up to date (check in with your doctor every few years to see if what you want has changed).
I'm 37, and in good health. I have outlined the (pretty limited) scope of advanced care I would want in the event I became incapacitated, and have a designated healthcare proxy that I trust to make decisions consistent with my wishes.
This tempts me to leave similar instructions if I can't cook for myself.