I just came up with this idea. I'm not going to make this look nice. This is version 0.1 or something.
The Bimillennial Law, as it came to be known, was a peculiar thing. In 2103CE, it was noticed that life-extending technology was coming about so quickly that people just weren't going to die of old age anymore. It was realized that the overcrowding that this would cause could lead to great suffering, but it was hard to pick an age at which to put people down. 100 was clearly too early, since people had already lived several years past that 100 years ago. Some people thought that nobody should out-live Abraham, from Hebrew legend. Others used the age of Adam from the same mythos. Still others thought that nobody should live past 999, to limit ages to three digits - in decimal. Eventually, the age of 2000 was settled, being far enough away that nobody had to worry about it any time soon, and old enough that anyone who survived to it would have lived a satisfactory life, and could start to suffer from having to deal with just so many profound changes in a lifetime.
Later on - though well after the establishment of several extraterrestrial colonies - someone realized that this was stupid. The vote to repeal the Bimillenial Law was unanimous. Nobody was ever executed under this law.