My 4 year old reads at what the average dim bulb considers a third grade or higher level. Honestly, my parents and grandparents considered it about right for a pre-K, but standards have to be lowered to keep from hurting anybody's feelings, then as people barely live up to the standards, the cycle repeats.
^^ This. I was taught to read the alphabet at age 3, was reading The Hobbit by age 6, and Stephen King by age 10. Maybe not age-appropriate, but reading skills well above those of my peers in kindergarten. I was invited several times to join advanced placement programs and did them for a little while (while I was still an idiot impressionable kid from kindergarten through about 3rd grade), but bailed on them when I decided I didn't like the additional mandatory classwork on top of the standard course homework.
The parents that put their kids into programs like this make a time investment early on in the preschool years to cultivate a voracious mind, and that mind NEEDS to be fed once its appetite is woken up. Some people need the menu handed to them and do well in structured advanced coursework where the subject material is chosen for them, and other people apply that energy other more independent ways.