I skipped grade 4. There were about 8 of us in an organized program.

Contrary to what you think about 5 years in high school, I did high school in 3 because I could choose my subjects and I was already ahead in math and sciences.

Looking back, I was rather young and the drinking age was 18 then, so it wasn't the best thing, but I chose subjects that I wanted, rather than thinking I should hold myself back.

Your child may not want to "decelerate" in high school, just for the social reasons. They will probably want to move along with their friends.

Ren