A local private college prep school did a longitude study of its students for the last 30 years looking at these questions and others.
They found no difference in age at start of K/1st, skips, and outcomes. And IQ was only part of the equation when it came to high grades, test scores, and entrance to great colleges.
They do not yet know what is "too young" but skips of 1-2 years and getting a bachelors at 19/20 does not appear to be a negative at all.
Divorce is the biggest negative factor they have seen.
If one considers all of those things when choosing to skip a grade or graduate early, I don't see why being younger should be a negative in and of itself.
I know of someone who is 19 and in med school. They skipped two grades and graduated at 16. Then did the BS in 3 years Summa.
