I used a few methods, but none of them are like dieing from old age.
During a heat wave (or a vey hot day) you can dress a sim in winterwear and go outside. The sim will die from over heating.
On a freezing cold day you can dress a sim in hot wear and go outside. The sim will freeze to death.
Both these methods are rather quick, but you will need to watch the to stop them from leaving to help their needs.
These days I simply set their hunger to zero (I use mod UI Cheat for this), and they will fall to the ground immediately. Death by starving will follow them in your family tree, but hey-ho, who cares ;)
I wish there was an easy way to do it.
I also wish there was a way to have birthdays without eating all those cakes...