If you are still running Oracle APEX 5.1, it might be time to schedule an upgrade. Support is running out at the end of the year.
Does it mean you can't use Oracle APEX 5.1 after December 2011? No, not at all, if you have no need to upgrade, and all is running smoothly, it's fine... you just can't request a patch anymore.
Till the 20.2 release you could stay on an APEX release for 5 years, but that has changed since the 21.1 release. Oracle announced it will reduce the support for an APEX version from 5 years to 3 years.
As Oracle APEX is a web development tool and things change so fast on the internet, I would recommend upgrading faster than every 5 years anyway. From a security and browser/mobile compatibility point of view, upgrading makes all the sense. Having said that, if you run an app internally and don't upgrade browsers within your company, I can understand if you just keep what you have and don't want the hassle of upgrading. Upgrading Oracle APEX is typically a breeze, but still testing is required and some things might behave slightly different or became deprecated.
At United Codes we keep 3 instances on our main server: Oracle APEX 18.1, 19.2, and the latest version of APEX, to which we upgrade very fast. The 18.1 release we use as a base for most APEX plug-in development, unless we need some specific plug-in features for which we need to use 19.2 (e.g. for IG support).
With APEX Office Print we try to support all features of APEX, e.g. when the new maps component came out, we made it so you can include it in your documents. So for that development, it's crucial to stay on top of the game. To most of our customers, I give the advice to wait for the first or second patchset, depending on our own findings.
I wrote this post for Joel Kallman Day, a day we push out content as a community and try to generate a bit of buzz.
