I started writing this post and researching while I was under the impression that we didn't make mistakes or even lost money when we were dealing with youngsters. It's a fact that most of the time we sold u-23 players for more money than their new club got for them later. However, we did sell some youngsters for less than they were worth later. These are all the transfers of u23 players from the last 15 years, which were sold by Juve for less than what they are worth now, or for what their new club sold them later.
Perhaps one day De Ligt can be added on this list, while so far it's like this.
2008 - Domenico Criscito to Genoa for 11.500.000 --- In 2011 Genoa sold him to Zenit for 15.000.000
2013 - Ciro Immobile to Torino for 13.500.000 - in 2014 Torino sold him to Dortmund for 18.500.000
2013 - Manolo Gabbiadini to Sampdoria for 11.500.000 - in 2014 Sampdoria sold him to Napoli for 12.500.000
2013 - Simone Zaza to Sassuolo for 10.000.000 - in 2015 Sassuolo sold him back to Juventus for 18.000.000
2013 - Luca Marrone to Sassuolo for 4.500.000 - in 2014 Sassuolo sold him back to Juventus for 5.000.000
2015 - Domenico Berardi to Sassuolo for 10.000.000 - still in Sassuolo, but could be sold for much more than 10.000.000
2016 - Gabriel Pires to Leganes for 2.500.000 - in 2018 Leganes sold him to Benfica for 9.600.000
2017 - Kingsley Coman to Bayern Munich for 28.000.000 - worth much more nowdays
2017 - Pol Lirola to Sassuolo for 7.000.000 - in 2020 he was sold to Fiorentina for 11.000.000
2019 - Moise Kean to Everton for 27.500.000 - in 2021 we paid 7.000.000 for the loan, in 2023 we paid 30.000.000 for full transfer
2020 - Cristian Romero to Atalanta for 19.320.000 - in 2022 he was sold to Tottenham for 50.000.000
Romero and Coman are the main mistakes, I believe. Immobile... I don't know. Berardi, maybe. Criscito, Gabbiadini, Marrone, Lirola, I don't consider them mistakes at all.
It's yet to be seen about Rovella since every case is different, but the past doesn't really prove that we tend to make terrible mistakes when we sell youngsters. In at least 3/4 of the cases we were selling well.