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I recently flagged a closed question for moderator attention because it was off-topic and would have been better suited to another SE site. However, my flag was declined, and I was told "it's too old to migrate". So how long is a question eligible for migration? I can't seem to find the rule for this. I would like to know so I don't waste moderators' time in the future by filing impossible requests.
Note that the fact that the question was already closed is important here. Normally, if a question is off-topic, I would choose an "off-topic" flagging option. If the question can still be migrated, there is an option called "This question belongs on another site in the Stack Exchange network" under "Flagging > Closing > Off-Topic". If the question is too old to be migrated, this option doesn't appear. However, if the question is already closed, the entire "Off-topic" flagging menu is unavailable because it's part of the "Closing" flag menu.