Your Sims' life doesn't end after their retirement. They will not be disappeared, but will still stay as your elders. Elders are similar to your grandparents. They are the oldest member of your family. In The Sims Mobile, elders are Sims with maxed out traits. They cannot consider as elder just because of their longevity in the game.
Once you've decided to retire your Sims, they will no longer be playable, but you can still interact with them and establish stronger relationships. Your playable Sims can earn XPs if you interact with them. They will function as your grandparents that will teach you often about life and everything they know. That is what we called "Mentoring".
After a few weeks, your retired Sim will still bid goodbye and leave. This is the hardest part since you've established a strong bond with your elder, they will still decide to go on a separate way.
What is "Mentoring"?
Mentoring is a means of supporting and encouraging someone. It's a successful way to aid a person to move forward and develop in his/her chosen path.
In latest installation of The Sims Mobile, this social event was introduced. Mentoring is an interaction between your playable Sims and your Elder Sims. It is an event where seniors are passing their knowledge to the young ones and allowing them to earn Hobby or Career XPs.
The sole requirement to have this kind of social event is to have an Elder Sim. For you to have an Elder Sim, you must retire your playable Sim. Please take note that you can no longer control retired Sims, in addition, retiring a Sim will give you a Heirloom. Click here for more information about retirement.
I'd like to know the same thing. It has been happening since the Update. I've had 3 sims leave no Heirlooms even though they've gotten married, had kids, and completed events and jobs/hobbies. I'm getting eally irritated