Being advanced(PhD) or gifted usually means pursuing more open-ended type interests and research. The foundational skills, which form the majority of K-12 curriculum, won't be out of reach for someone keeping pace in non advanced tracks.
Where I live, you get tested whenever your parents want you to be tested, and if they pass the advanced learning at the point, they can choose to elect for the 'advanced school(s)'.
_edited for clarity_
...which doesn't apply when the "track" is a set program at a different school.
Jumping in and out of the "gifted" track should be possible if such a thing exists; however to your point, how would the track be defined and what should it contain? Would there be a list of items that need to be mastered? What evaluations determine if you can jump in... or when you need to "jump out"? Would the "gifted" track be age based the way it is currently or could the contents be tailored to fit the needs of the individual students?
Following that line of thinking, at what point do you qualify for a job? Also, what social stigmas would we attach to "jumping out"?
I don't have answers, but your point led me to some very interesting questions. Thank you!
The problem is if they've missed the first eight years of the gifted track, they can't just transfer in and be expected to know what's going on.