Defensive chemistry is a key plant fitness trait, and the investigation of the expression of plant secondary metabolites across life stages is important in understanding the lifetime evolutionary selection pressures on a plant. The expression of genetic-based differences in foliar defensive chemistry, known to influence mammalian herbivore preferences, was studied across two contrasting life phases of the heteroblastic tree, Eucalyptus globulus. With plants from different subraces of E. globulus growing in a field trial, we compared the levels of seven chemical constituents in adult and juvenile foliage from related coppiced plants. Defensive chemistry was generally higher in more vulnerable coppice foliage than adult foliage. Significant, genetic-based differences among subraces were detected for two key defensive chemicals, a sideroxylonal and a macrocarpal, and these differences were stable across life phases. In contrast, significant differences among subraces in adult leaf condensed tannins were not evident in the coppice because of the absence of this group of tannins in this foliage. These findings lend support to hypotheses that suggest condensed tannins may have evolved for reasons other than mammalian herbivore defense.