The life cycle comes to a close there, as that Sim will die of old age. They will age up into full adults, and finally into their Elder years.
Sims go through various states from baby, on to todder, childhood, through the teenage years, and finally young adulthood. The Sims 3 features an aging system not unlike that of reality.
The Sims 3 Aging and Lifespan Guide Stages in The Sim Lifecycle, Settings, and Aging Down or Up Related Links