According to the radio piece I heard, the authors recommend that those 13 - 40 should be the first for vaccines based on 2 principles: 1. everyone should have the opportunity to live through an entire lifespan (thus don't save the elderly, they have already had their chance); and 2. those who have already an 'investment' in life should benefit before those who haven't (thus don't safe the young).
Funny how the very young and the very old (the weakest) are always the ones to be shifted aside for the benefit of others.
But the fundamental problem with this philosophy is the propostion that some lives are somehow more valuable than others. Don't even save the most people, save certain segments. Today it is the 13-40 age group, tomorrow it is the group with the right ethnic background and political or religious beliefs. (Disclaimer: the whole value of vaccinations in general is an entirely further discussion.)