Cities: Skylines II

This is easy and can be achieved pretty quickly; any decent city will do it. Keep an eye on the happy icon toward the right of the icon row at the bottom of the screen and address any issues reported there.

Understandably, citizens like low-ish taxes, parks, good services (medical, police, fire, etc) and having education and jobs. Standard stuff, really.

Per the in-game notes on the achievement - this does need to be in a single city.

The requirements for each unique building are clearly listed in the game, as is whether each has been unlocked/built - so this is easy to track.

However it will take a while as some buildings require hundreds of regular buildings to have upgraded to level 5 - so your biggest enemy will be simulation speed slowing down causing this to take forever as your city inevitably grows.

Its probably best to leave the (easier) regular residential ones to last and focus on commercial/industrial - either way it'll take a loooonnnng time. Almost as long as You Little Stalker!

You'll need this on the way to Key to the City - signature buildings are in the icon next to zoning. Each one has a different requirement for your city to be able to unlock it; the tooltip for each will explain what the requirement is; it'll be either X units of a certain zoning type, or & buildings levelled up to level 5 or similar. The first 5 (any 5) are pretty easy; pick the zoning ones.

Technically this is optional, but in practise it'd be nigh on impossible to miss this during the course of building out a city. You'll get it pretty quickly.

This is Residential, Commercial, Industrial and Office. This is easy to get as soon as your city is large enough to be able to zone the different types.

More map tiles can be unlocked as your city grows and passes the various milestones. 50 tiles isn't too many and will be easy enough to get as your population grows.

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...