Foliage-age variation often affects the distribution and abundance of specialist herbivores, but its influence on generalist herbivores remains poorly understood. We carried out field surveys and manipulative experiments to investigate the foliage age preference-performance relationship for a generalist caterpillar, the whitemarked tussock moth, within balsam fir trees in eastern Canada. Young caterpillars preferred and had the highest survival on developing foliage. In contrast, older caterpillars fed on both developing and mature foliage and had the highest overall fitness when allowed to feed on a mix of all age-classes. This study supports both the ‘balanced nutrition’ hypothesis, which states that dietary mixing of different-aged foliage can increase nutrient uptake while reducing uptake of secondary chemicals, and the ‘ontogeny’ hypothesis, which attributes changes in foliage-age preference to changes in the dietary needs and/or tolerance to plant defences of juvenile insects as they develop. This study demonstrates that the selective pressures resulting from large age-related variations in foliage quality within conifer can influence the fitness and feeding preference of a generalist in ways similar to that previously reported for specialists.