That's one thing this otherwise excellent article didn't take into account. Many of the homeschoolers I know got their GEDs at age 15 or so, and are in community college taking courses by age 16.
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Yes, homeschoolers often take the SAT and ACT at far younger ages than institutionalized children.
By the way, two of my homeschoolers finished college at the age of 18 with B.S. degrees in mathematics!