I'm 72 with slowed capacity. I have had similar thoughts. Even if you agree with the idea, implementation is tricky. Let's say I stop all life lengthening treatments at 75. Well, I take warfarin blood thinner to prevent clots from forming on my mechanical heart valve. These clots could break off to give me a heart attack or stroke. Clearly this is a life lengthening treatment. Should I stop taking it? If I stop and a clot develops there is a chance the the ensuing heart attack or stroke could leave me deeply crippled, either mentally or physically, but very alive -- to spend my last decade(s) in bed.
In biology there are no sharp lines. When does a child become an adult? 16? 18? 21? 25? All of these ages could do. In the same way, Emanuel, if he does not change his mind -- as most people I know do, will have great difficulty making these decisions except in extreme cases.