I have retested students at around 17 or 18, with access to scores from primary age, and also find that scores can vary a fair amount, in either direction. I had one student rise from being classified as intellectually impaired to solidly average overall (but with a specific learning disability in mathematics/visual spatial, which I speculate was among a list of circumstances interfering with early testability in this student). So a change of about +30 points. I also have seen scores fall from 130+, generally down into no less than the 110s. Typically, those scores start slipping earlier, and have been gradually declining over a decade or more. The nature of decline is often associated with their areas of disability. My ASD students tend to have scores rise gradually. Keep in mind that most of my experience is with students with disabilities, so these anecdotal general observations may not apply as much to GT without a second exceptionality.
I've also retested a couple of GT young adults without disabilities, with early scores in the 145+ range, one of whom came out exactly the same, and another of whom fell to the 130 range (though it should be noted that the first was tested on the same instrument family both times, while the second was initially tested on the SBLM, and later on an edition of the WAIS, so the likelihood of score stability was not as great).
