Generation Ship
Generation Ships are large, self-sustaining environment ships which allow for very slow interstellar travel at sub-light speed.
The first Generation Ship left the Earth in 2097. Up to 70,000 in total left the Sol system.
Known settlements from Generation Ships
The Phekda system was populated by one of Generation Ships.
Where to look for Generation Ships
From "ELITE DANGEROUS LORE : GENERATION SHIPS": [1]
"... the conclusion that many of the ships should be reaching their destinations around about now if you assume they were targeted at stars within 100 lightyears (which seems plausible for 21st-24th century technology). The maximum distance they could realistically travel would be around a 1,000 lightyears (probably much less)."
Also it mentions The Golden Age of Exploration which took between 2200 and 2700.
So it seems Generation Ships were launched between 2097 and 2700 (that is ~1200..600 years ago).