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There is some truth to the "this team is on a downtrend (due to age)" quote as Croatia was the 22nd oldest team out of 32 at the World Cup. I haven't checked whether some changes have already been made, but their average age is relatively high and new players will indeed have to be integrated soon. Not every player is as crazy as Modric and can keep that level even at the age of 37.
Average age can be quite misleading. Because you can have half a dozen 18 and 19 year old players in the bench and that will give an impression that the team is very young. There is no doubt that Croatia during Qatar 2022 had one of the most aged squads for any team. But to their advantage, they have a number of promising younger players emerging, who can replace these senior players. The situation with Croatia is different from what we have with Spain or Portugal, where the younger players are struggling to perform consistently.