The first release of the German Ph@ttSessionz speech database contains read and spontaneous speech from 864 adolescent speakers and is the largest database of its kind for German. It was recorded via the WWW in over 40 public schools in all dialect regions of Germany. In this paper, we present a cross-sectional study of f0 measurements on this database. The study documents the profound changes in male voices at the age 13-15. Furthermore, it shows that on a perceptive mel-scale, there is little difference in the relative f0 variability for male and female speakers. A closer analysis reveals that f0 variability is dependent on the speech style and both the length and the type of the utterance. The study provides statistically reliable voice parameters of adolescent speakers for German. The results may contribute to making spoken dialog systems more robust by restricting user input to utterances with low f0 variability.