You've received great resources in the post above.

I'll just add that the information which is helpful for "radically early college entrance" may depend upon the needs of the child and/or what you, as the parent, are trying to accomplish.

For example, some may seek a degree of celebrity. There has been much media coverage of Tanishq Abraham, easily found with a web search, such as this article from 2015 and this article from 2016.

There are also articles such as 10 youngest colleges grads of all time, featuring other students.

However the more common stories may be those of an 8, 9, or 10 year old auditing a college course in a subject of intense interest with a parent sitting at the back of the lecture hall or waiting in a nearby hallway.

Students taking dual courses (which count for both high school and college credit) are quite common.

Those who've completed high school and are fully matriculated college students at 15 or so are not uncommon.

I'm linking this thread (currently found in the "Parenting and Advocacy" forum) to a thread on early college in the "College" forum. This is to underscore the importance for parents to realize that once a child attends college, parental rights under FERPA end:
Parents should understand that their rights transfer to the student when he or she reaches the age of 18 or attends a postsecondary school at any age. Students to whom the rights have transferred are "eligible students."

As for advice, my advice would be: Never play the age card. In other words, do not ask for special favors, support, attention, or consideration due to age. College is more than pure academic/intellectual pursuit. An amount of maturity which affords the ability to understand the system and work within it (essentially developing internal locus of control, and demonstrating positive self-advocacy) may be hallmarks of a college-ready student. A child may need practice to become adept at deflecting conversations from the topic of age, and/or redirecting the focus of conversations from age back to the topic at hand (in a nonchalant, ho-hum, matter-of-fact manner) if someone else should veer off-topic and probe about age. Others may be curious, and that is to be expected, however the twin goals may be to avoid becoming a spectacle, and to maintain composure.

Without revealing identifying details, would you care to share a bit more about your circumstances which lead you to inquire about radically early college entrance?