I ran into "off topic (too old to migrate)" on the close dialog when attempting to close-vote this question. What does it mean?
Questions that are older than 60 days cannot be anymore migrated; not even moderators can migrate them. What that phrase says is that you can just close the question as off-topic, but you cannot anymore vote to migrate it.
The reason for stopping the migration of too old questions is that they are problematic. Often, they are closed in the site where they get migrated; even if they are not closed, they don't get much contributions. This is especially true for questions where the top answer has a high score; even if somebody would write a very good answer, it could not emerge from the answers given in the original site. In the case the question doesn't have answers with high scores, it is probable that the receiving site has already a similar question (although not a duplicate), and users probably aren't interested in a very similar question again. It could be also the question is outdated, and nobody is going to add a new answer.