I am browsing the planespotters.net database, and I see plenty of planes in their twenties. Delta, for example, has 117 MD-88 with an average age of 25 years.
While other planes are scrapped with no apparent reason when they are not even 10 years old: when Malev went bust, all its B736s were scrapped, 6 planes that were around 8 years old. https://www.planespotters.net/production-list/search?fleet=Malev-Hungarian-Airlines&manufacturer=Boeing&subtype=737-600&fleetStatus=historic
This is not the only example, there are plenty of aircrafts not older than 15 years that are scrapped, while other planes are still flying in the late twenties.
What are the reasons?