At some point in your game in The Sims Mobile you will see a message. The game will tell you that your Sim can already retire. You can agree and your Sim will automatically leave the family, but you can also wait for this choice. This message will appear even if your Sim has never worked or developed a hobby.
If you don't give a consent to the retirement of your Sim, this message will wait for you in the "Features" section. You can click and send your Sim to retire at any time. However, if you make a party, the message will disappear and you will have to wait for the next one.
When your Sim retires, the rest of the family will stay at home. They will not feel the lack of this Sim. Hobbies and work can be started by a different Sim at any time. However, he will start his career from the first level.
Retiring Sim leaves a random souvenir behind called Heirloom. These unlock new features or hobbies.
A retired Sim will stay at your home, but he/she becomes an unplayable character. In order to move your retired Sim out from the house, you can enter the generation panel (as shown in the screenshot) and Move Out your Sim from the house. This process is irreversible.