A new study in Nature this past month indicated that the teenage brain is fairly plastic; that IQ can change a reasonable amount during those years of rapid physical change (You can read one write-up of the study here).
As usually happens, some of the commentary on this study has raised the question of whether giftedness exists, or if it just captures a moment in time. But I think this misses the point -- while IQ may not be absolutely constant, a child whose IQ measures at, say, 150, is unlikely to then measure at 100. And just because IQ can change doesn't mean that giftedness shouldn't be accommodated. Gifted education is an intervention for children who need it. If one doesn't view it as a reward, then there's no reason children couldn't move in and out of needing services over time. It's something to consider.