My understanding was the CDC/FDA guidelines said the age for starting, that it's perfectly fine for finishing the series after the maximum age. I could be wrong, I've never actually given one of these things.
My other understanding was, there is nothing harmful about giving these vaccines after the age of 26, it's just it's probably unnecessary at that age because the person has probably already been exposed to HPV. The same reason we don't give Rotarix to adults. It only makes sense to finish the series if someone has started it, regardless of age, assuming they have they have the money/ins and desire to finish it.