Questions older than 60 days can't be migrated by moderators. The way this is currently implemented in the flagging and closing UI is that the option "This question belongs on another site in the Stack Exchange network" is simply missing from the dialog.
I think it would be better to disable the option and replace it with a text explaining that the question is too old to migrate. Users that are not active on meta often don't know about this restriction and might get confused. And even experienced users sometimes forget about this restriction if they don't pay attention to the age of the question.