I'll have to repeat, please read How do I ask a good question?. The question is too vague - why would you ever consider using old JDK or PostgreSQL versions? What "risks" are you talking about? There shouldn't be compatibility issues (unless you use abandoned libraries) and the old versions won't have any of the fixes and performance improvements introduced in the newer versions.

Databases especially live for decades, while upgrading them is non-trivial. Quite often it's better to migrate to new iron instead of trying an in-place upgrade. And PostgreSQL 18 introduced significant performance improvements especially around IO. At the very least, don't use an older database, to reduce the need (and risk) of migrations.

As for Java, do you have a specific library that somehow only works in the 2-year old Java 21 ? In that case the real problem is the library, which is probably abandoned. Or the newer version wasn't purchased for whatever reason.

With Java at least, it's a lot easier to migrate once you get rid of obsolete third-party libraries. Breaking changes are very rare and announced well in advance, allowing developers to upgrade their code. If you feel Java 25 (released Septemeber 2025) has teething problems you can start with Java 24 (March 2025) and upgrade later.