Under certain worldviews (the ones that say that execution is a punishment carried out for the benefit of the one being punished, rather than exclusively for the benefit of society as a whole), if he had lost his mental faculties, there would be no point in executing him.

... that as it's a punishment, it should be given to someone who is aware that they are being killed because thay have done a bad thing.

But I agree, the world generally has a view that doing things to Old people is particularly bad- so if a pensioner gets mugged, that's a bad crime, although mugging a 20 year old is just one of those things. Funny.