Let's pretend there's a question asked on Stack Overflow (as if they ever are). The question is asking if there is a better way of doing something. Clearly the question belongs on Code Review. You go and click the "close" link only to be greeted by the words "(too old to migrate)" next to the "off-topic" selection. You look at the question's age. 2 months. You look at the view count. It's around the 100s. "That's kindof low" you think to yourself. Finally, you look at the vote count:
A question belonging on Code Review that is too old for migration that has a low view count and a score of 1. Why can't it be migrated? It's older than a month, but that doesn't answer the question. Why can't it be migrated? Because it's "too old":
Yes, "too old" is a good reason not to migrate. Old questions are of very low value, and when old questions go from SO (with the matching crazy high views/votes) they're very disruptive to smaller communities: piles of free rep and badges for the owners of the migrated posts, the question looks disproportionately good via upvotes, the tone of the question/answers are from another site...
Hmm. So the reasoning is that it's disruptive and the many badges and rep from votes are unproportionate. That doesn't sound like a reason to prevent questions from being migrated just because they are old. It seems that the real reasoning is a (ridiculous) precaution against questions garnishing a lot of activity.
If that's the case, why not decide migratability based on the amount of activity the question has received. Maybe we could allow any question to be migrated in the one month period, but after that, determine migratability based on activity?