Someone that peaks too soon. Coined in the Mike Meyers, Gwyneth Paltrow movie View From the Top. Used to describe someone that reached their peak too soon, and are headed towards a steady decline in skill, intelligence, etc.
Gwyneth Paltrow: How could I have failed that test? I am at the top of the class!
Mike Meyers: Well...you're a peak-too-sooner. You peaked too soon.