With that said let's look at the average age of Klopp's Dortmund teams compared to the rest of the league and where they finished.
2008-09: 25.8, 8th youngest team, 6th position
2009-10 24.4, 3rd youngest team, 5th position
2010-11, 23.8, 2nd youngest team, Champions
2011-12, 24.3, 3rd youngest team, Champions
2012-13, 24.5, 5th youngest team, 2nd position, Champions League final
2013-14, 25.5, 10th youngest team, 2nd position
2014-15, 26.2, 13th youngest team, 7th position
You'll notice that in the two years Klopp won the league with Dortmund, he had the second and third youngest teams in the league. When they reached the champions league final, they were 5th youngest. Those were teams with the legs and physical ability to carry out his pressing demands to the level his preferred style of play needed.
Now let's look at the average age of his Liverpool teams. We'll leave out the 2015-16 season since he wasn't hired untill October.
2016-17, 25.6, 2nd youngest team, 4th position
2017-18, 25.5, 2nd youngest team, 4th position, Champions League final
2018-19, 26.2, 8th youngest team, 2nd position, Champions League winner
2019-20, 26.6, 6th oldest team, Champions
2020-21, 26.8, 5th oldest team, 3rd position
2021-22, 27.7, 3rd oldest team, 2nd position, Champions League final
2022-23, 27.0, 3rd oldest team, TBD
He's managed so maintain some level of success even has team ages this time around, but it loses its consistency. For example, while last season was a success, the year before that, they kicked off their season with a 7-2 loss to Aston Villa in a game where Virgil Van Dijk, Firmino, Salah, Robertson, and Trent all played and needed a last gasps header from Allison to qualify for the champions league.
This Liverpool team, as currently constructed, is too old to perform the physical demands Klopp's playing style needs, which is a good thing. That means with a refresher of the squad with much younger faces, one of the greatest managers in history will be back at his best.