Death is one of the inescapable part of being a human. Aging is also included since we can't remain forever young. We will eventually grow up and become matured. Death may come unexpectedly because of accident or disease. It may also occur naturally as we grow old.
In the Sims Mobile, Sims also experience this natural process in a different way.
Sims will stay fully grown adult once you created them. They will not grow old like us. They are not aging for each day or year they have existed in your game. The only kind of Sim that will age and grow is a baby Sim.
For the Sims to age and be mature, they should perform interactions with the other Sims and build relationships. They should gain and hone their skills by executing and completing events. Their traits will be the factor of their adulthood.
Sim's retirement from the game is now in the player's hands. In the former version, elder Sims have only two days before automatically retiring. This gives you a disadvantage to step upward with your Sim's career and hobby; hence, limiting your ability to unlock objects and other stuff.
Now, once you've reached player level 16 and max out your Sim's traits, you will receive a prompt to retire that particular Sim or not.
The retirement option will be available in the Trait menu of that said Sim. Retired Sims will leave a Heirloom. These can unlock new traits, levels and other bonuses.
NOTE: If you missed or disregarded the retirement notification, you have to imaginary raise another trait to get the prompt or the option to retire. The game doesn't have a progress bar for traits. Basically, you need to play and pray to receive that option again.
TIP: To raise another trait, consider the traits that you have with your Sim. For an example, if you have "Kind of a Big Deal" trait that boost your DJ Career, assign your Sim to be a DJ and perform lots of DJ career events.
Retired Sims will not be gone forever. They will still be around, but not playable. They can mentor your current Sims to earn XP. Mentoring is passing the knowledge of your retired Sims to your current playable Sims. That is similar to your granny scolding you about life and experiences.