What is the scope of this site - in terms of how old something has to be before it is history?
I would say that the length of time is mostly irrelevant. Far more important is if the question touches on topics that are still "in-play". For instance, asking on Egyptian military's role in what is termed the "Arab Spring" is tricky as the Arab Spring is still developing in Egypt (its still hasn't passed the deadline for the military to hand over power to a civilian government). That is a good instance where the question should be discouraged as what happens in June will be a major factor in determining the answer.
Now if the question was to ask "what role did the Egyptian military play in developing the Arab Spring popular demonstrations in Egypt", this would certainly be a valid question as, even if events referenced in an answer happened say within the last 10 years, the point at which the Arab Spring started is not dependent on something happening in the future.
That is up to the community to decide. Try asking your question and see how everyone responds to it.
I think that the closer you are on the timeline to your own time period the more likely it is that the "history" has not settled. Similarly, the further away you are from the events in question the more likely you are to find some sort of consensus.
So things like the Arab Spring, which BrotherJack pointed out, are still very much open to debate, but the historical conditions that led to it occurring would be an easier question to tackle.