I'll chime in as someone who was not radically accelerated, but who made choices that allowed for some acceleration in university.
My public elementary school volunteered a skip from 3 to 6, but my parents were concerned about socialization and encouraged me to select a single year skip. At the time, gifted testing began at the beginning of fourth grade. I moved into the 5th grade gifted program 6 weeks into the school year after the test results came in. I was not academically challenged, made friends, but felt like an isolate, and spent most of my time reading in class.
I moved to a private girls' school for 7 to 12 with a group of university bound girls. The social dynamic was terrific, and I quickly morphed into a social butterfly-hyper achiever, grew to hold leadership roles in many clubs and played several varsity sports. I had an A+ average every year, despite never doing work outside class and goofing off in class on my laptop with friends.
I managed to cut the final year off high school, complete the IB diploma program, and gain credits for first year university, giving me second year placement while preserving first year scholarships. University was probably the first time I'd ever had to work, but after a semester I had figured out how to get As with minimal effort while enjoying a robust social life. I enrolled in a master's program, compacted and completed it in 8 months, and went on to work as a researcher at a think tank at 20, where most of my colleagues were 30+.
I'm by no means as accelerated as many folks here. In hindsight, I think any number of arrangements could have been successful, including another year or two of acceleration in elementary. The single sex environment was, for me, a social catalyst. I went to university between 3 and 4 years younger than peers and blended in well. In my first year, most of my friends were seniors.
Just thought I'd tell a story of self-governed, late stage acceleration to show that the game isn't over in elementary.
