As having 10K+ reputation I do flag review on regular basis on Stack Overflow. Today I saw an very old flag approx 7 months old, here is a snap shot,
Just about half an hour it was not showing there and suddenly it started showing. It is a Not an Answer flag of this answer.
Generally I notice that flag takes approx 24 hours to handle, then how come this very old flag is still in the queue ? About half an hour it was not there.