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...
For an example, let's look at Difference between UI and UX (10k only), an old Stack Overflow question migrated to User Experience. The answers are from programmers, and it shows. It really really shows. The top voted answer is a couple of links to Wikipedia and a "Well I guess it means this". As a mod I've strongly considered deleting this question and starting over.
There's not much you can do with an old question. The asker has probably moved on, the answerers don't care, and all the voting in the world (practically speaking) won't fix that terrible answer being highly upvoted and accepted.
Note the "yes" argument is from a 3 year old question when the network was considerably smaller. The mood seems to have settled much more toward letting communities organically grow, and tossing old questions, especially to Beta sites, doesn't help them do that.
@amanaPlanaCAnalPAnaMA: Quoting @DavidZaslavsky (cannot find link, from memory) "The existence of another SE site does not change on-topicness of a question". Sites can have overlap regions (as a Chemistry mod, I've already had to deal with these with Physics and Biology). The general rules for dealing with these is that they be moved to wherever they shall get a better answer (unless the tone of the question clearly tells you which site it ought to be on). The tetris one has some good answers (though @pekka hasn't accepted any). It won't be helped much from migration.