I've wondered about this. There's a continuum (or several continuums) of abilities, and any arbitrary cutoff is arbitrary.
Some things are true about different educational needs, such as, more intellectually able kids need more advanced material (maybe 1 or more grades ahead of their age), faster coverage, less repetition, less practice, deeper coverage, more challenging problems, and so on. Again it's a continuum. These educational needs don't just switch on above a certain arbitrary cutoff.
But as an immigrant to the USA who wonders about this, it seems there's a lot of history and politics to this whole gifted thing. Many countries have "tracking" (or "streaming" or "ability grouping"), where students of different ability levels are put into different classrooms or different schools, so within any classroom students are at similar levels, and the teacher can teach at the right level for all students (except maybe those at extreme highs or lows). For various reasons the concept of tracking has met with viscious opposition in the USA. In the absence of tracking, parents and students try to figure how they can carve out some niche where they can get a more suitable education, but they may have to be careful to disguise it as something other than tracking, to evade attack. This gives the imperative to the idea that "gifted" students are in a "special needs" niche.
