My 11 year old son went into Kindergarten at age 5 reading at a 5th grade level. His favorite book to read at the time was a science encyclopedia. By the time he turned 7 it seemed like he could read anything but it was comprehension that he needed to work on. His comprehension also increased quickly because he loved learning the meanings of new words, especially words with multiple meanings, because he could use these in the jokes he made up. He got into the habit of looking up words that he heard but didn't know the exact meaning of. He had to because I didn't always know the exact meaning. He also started reading classic books with lots of rich vocabulary several years ago.
It wasn't a problem for him to do this because he homeschooled after Kindergarten.

I live in a state that doesn't require testing of homeschoolers and I couldn't really afford private testing. The only testing my son had until recently was the WIAT at age 7 and that was because insurance paid for it.

I had him signed up to take the Explore test once but he had a cub scout event on the day of the test that he didn't want to miss and I didn't want him to miss out on something fun. I would still like for him to try the Explore test before he has to take the ACT since he has very little testing experience.