If you only immunize 50% of each generation against a virus, then you slow the transmission rate of the virus, but you also allow the number of susceptible individuals to grow over time. Eventually, the number of susceptible individuals reaches a critical threshold, and outbreaks occur.

Because this process takes years, by the time the outbreaks begin, the average age of the un-immunized population has increased. On the other hand, if nobody is immunized, most people will acquire the virus at a young age, become naturally immune (or die), and the average age of incidence will be low.

Here's a link to a slideshow that explains the concept in greater detail (I've started it at a slide that discusses the Greek rubella outbreak, but if you want the mathematical models, go back to slide 1):

http://www.pitt.edu/~super1/lecture/lec1181/023.htm