Why is redshirting so rare if it's so advantageous?

“In my case, I was 16 for a little while my freshman year of college and just didn’t tell anyone. I received an academic scholarship and graduated in three years.”

I’m confused. You say you were 16 at the beginning of your “freshman year of college”, but wouldn’t you have had to have started as a sophomore in order to graduate in 3 years? Then again, maybe it the case that you had graduated high school with just under a full year’s worth of credits(Maybe 59 units) and you were easily able to make up that extra unit.