I know that there are three reasons with which a flag can be dismissed: helpful, declined, or disputed. However, I recently came across this fourth one:
It has been pointed out to me that this could be a bug. What causes flags to be dismissed with it, and when will this be fixed (if it is a bug)?
It's a bug in the rendering of that specific flag type.
After a period of time, flags that are not acted upon by mods or other community members will get cleaned up automatically by internal scheduled tasks. This prevents flags from accruing over time and allowing actions that the greater community in general doesn't want.
This cleanup process can also be triggered by review tasks that contradict a flag. This is what occurred (erroneously*) in your specific case.
Next build will fix the display to read just that: aged away.
* I agreed with your flag - that question didn't belong on SO (or anywhere else), so I deleted it. A simple Google Search would have provided its answer.
