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. If a Sim achieves it before becoming an elder, the aspiration bar will rise to the new maximum at the top of the hour after the Sim has aged up. Even if the Sim fulfills a major fear, their aspiration bar will dip, then return to maximum at the top of the next hour. 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 reportedly changed with the introduction of FreeTime and lifetime aspiration. Reportedly, a Sim who fulfills a Lifetime want in FreeTime achieves maxed-out platinum mood that decays very slowly. In practice, many Lifetime Wants, even those for FreeTime careers, still refer to the Sim receiving perma-platinum, and it appears that fulfilling Lifetime Wants still grants it, though there may be individual exceptions.
FreeTime did introduce two new ways of achieving perma-platinum. One is to max out the Lifetime Aspiration bar. The other is to use a Genie to make the wish "Give Me Peace of Mind". If successful, the Sim receives Perma-Platinum instantly, but this may not be reflected in the aspiration bar until the top of the next hour. If the wish fails, it takes about one game hour to take effect.