Childhood and adolescence spent waiting for others to catch up was excruciating, but adulthood has been a disappointment, too, because I never considered the fact that while I was "waiting" for them, I was not standing still either.
But PG people are at an extreme of that extreme. In addition to what HK describes of being ahead of my peers, I also experienced other kids being too quick to leave the interesting parts of childhood behind. When we hit junior high, play, invention, and mental exploration seemed to shut off like a spigot....
The concept of "mental age" as a way to understand gifted development is a myth. It's not "ahead" or "behind." It's qualitatively different.