I agree with no5no5. Some gifted kids are clearly gifted as preschoolers (solving simultaneous equations? -- gifted) and some are not. (This also ties in to what Grinity said about the gifted group getting x% "more company" as the kids get older.) This may partly reflect the primitive state of our knowledge of how to test giftedness, but it may also be just a fact of development.
About "fizzling": One thing that may happen is that kids with a narrow skill may start to show their limitations as they get older.
For example, people tend to get very excited about musical or artistic talent at a very young age, but those kids almost certainly aren't really being artistic in the full mature adult sense. They may, for example, just be extremely good mimics of adult performance. If so, as they get older they may not mature as artists any more than any other kid.
Another example might be a kid who is extremely gifted in math, but only math, and maybe also has fairly severe 2e problems that don't become apparent until later. Such a kid may appear to the adults to have "fizzled."
In both these cases, though, I think it's not that the kid really fizzled, its that the adults developed excessive expectations based on the child's early skill.
