Our experience is different. (Actually we are home/virtual schooling, not using the B&M gifted schools, but that's because we decided we preferred home to B&M.)
Around here gifted identification starts as early as age 3, and there is free gifted education from pre-K to grade 12. About 3%-4% are identified through a combination of ability and achievement testing (should be >=2SD in one and at least close to that in the other), though you need to perform adequately in the classroom to keep the gifted designation. Students are in full time gifted-only classrooms, though the gifted schools are contained within regular schools, often in bad parts of town, for statistical manipulation purposes, which was a turnoff for us especially safetywise. Students can work several years ahead. I read an article about an 8th grader in our town who was taking AP chemistry because he had finished the high school math sequence already.
So, no, our "experience" is not the same as others in this thread. Early identification and multi-year acceleration are allowed.
We live in an area where there are zones of high poverty, high crime rates, and generally educational outcomes around here are abysmally low for the whole population in general. So they try to make up for it by having a good gifted schooling system.
I think the problem some of you are having is that you are in more affluent and educated areas, and in such areas, true gifted education gets stifled.
