You Little Stalker!
Ultra rare0.1%Follow a citizen's lifepath from childhood to old age.
The game has a follow mechanic where you can tag individual citizens to keep tabs on them, using the icon in the bottom-right corner of the screen.
To tag a citizen, click on them whilst they're walking about and hit the Follow icon in the info box that appears.
For this achievement, find a child and tag them to follow them. Then just wait until they reach old age.
Do this right at the beginning of a city, as quickly as possible - because it will take a really really long time for them to reach old age.
Your enemy here will be game simulation speed, so try to keep the city small and efficient so you don't lose too much CPU to simulating millions of citizens and waiting until you yourself die of old age before this unlocks...