Perma-Platinum is the state of nirvana that a Sim achieves when they fulfill a lifetime want. There are many lifetime wants, but however it is achieved, perma-platinum is a desirable state indeed. Once a Sim achieves it, their aspiration bar will remain maxed out until they die. Even if the Sim fulfills a major fear, their aspiration bar will dip, then return to maximum. This makes it easy for the Sim to maintain high job performance, as mood is always treated as being maxed out. It also means that the Sim will always be able to use aspiration rewards without risk of failure.
This was changed with the introduction of FreeTime and Lifetime Aspiration. In FreeTime, a Sim achieves maxed-out platinum mood when a Lifetime Want is fulfilled; however, the level will decay, albeit very slowly. Only when Lifetime Aspiration is maxed out do the Sims receive Perma-Platinum.
A Sim can also use a Genie to make the wish "Give Me Peace of Mind". If successful, the Sim receives Perma-Platinum instantly. If the wish fails, it takes about one game hour to take effect.