They won't last long. You don't last long in prison when you fuck with children, especially to the degree that these two did. I don't know where they are, but the crime happened in 2009 while the trial (and article) were 2011.
She's 20 in 2020 and the crime happened in 2017. What you're saying makes sense, but I don't believe the rules should change just because of the 18th birthday. It should be more on a case by case basis. Legally, at 17, she was a minor but she's not going to get new found maturity just by reaching 18 years of age. If she's mentally sound, she should be an adult for such major crime.