Depends on the kid and the local laws.
Here law says in school by age 7... Most people do K at age 6 (which creates a lot of problems in high school (18yo juniors, and 19yo seniors), but tends to help out a lot in elementary. Creates a HUGE age spread in our area; K = 4,5,6,7. 5th = 9,10,11,12. 12th= 17, 18, 19, 20... But MOST kids are 6 turning 7 in K (19 turning 20 in 12th).
I started my son in K at 5... And he was the youngest kindergartener in the school.
HERE, where we have a huge homeschooling population, homeschoolers typically start college at 15-16. College is free until they turn 18 or earn their AA or AAS or AS (2 year degree). Ivie are recruiting homeschoolers like crazy. There are a LOT of housing options these days on more and more campuses... Although most I know keep teen college students home (living or moving near their schools), there are a lot of underage options as well.