Sorry, but this sounds a bit short-sighted. While nobody should be sentenced to suffer severe pain for the rest of his life, just because your body isn't as capable as it used to be doesn't necessarily mean that you want to end your life. Often there will be people that you will leave behind and you certainly don't want to explain them that it's your decision to do just that.

On top of that, this is also dangerous from a social perspective where people could get a feeling that they are supposed to go once there is nothing they can contribute to society anymore.

We should rather focus on allowing making "regular" dying more dignified rather than promoting euthanasia.