that is very true, but then, most adult of that age would have most likely come into contact with it and built up immunity even if they haven;t had it.
that is not foolproof, they said that to me when I was pregnant and DD2 had chicken pox, yet I knew I had never had it. bloods confirmed it, plus also confirmed my lack of immunity, so like I say, being an adult is no guarantee of built-up immunity.
I would guess tho, that should there be an epidemic, any one who knows they have never had it, and so believe themselves at risk, could always ask about having it confirmed through blood tests and then be immunised if needs be.
don;t forget, they probably don;t have enough vaccines to also start on adults unless in dire need. especially at the moment when they are making tonnes of swine-flu vaccines.