Dr. porphyrin wants all of her female friends to get it, regardless of whether they're over 26, on the theory that it ought to be safe and may be helpful. I believe, from talking to her, that the age cap is there because 1) they've done trials with that age group because 2) it covers the highest-risk group, on the average.
I think it has to do to with statistical chance of already being sexually active by a certain age. If you've already been exposed to the virus through prior sexual activity, then your protection is reduced.
I'm pretty PO'd that my HMO isn't covering me for it because I'm "too old." Grr. Apparently they only cover it for people under 30 or somesuch. I'm probably going to pay it out of pocket if I can't get them to pay for it, though. :(
My understanding is that even if you've already been exposed to one of the strains, getting the vaccine does provide protection against the other strains.