After germination, plants progress through juvenile and adult phases of vegetative development before entering the reproductive phase. The character and timing of these phases vary significantly between different plant species, which makes it difficult to know whether temporal variations in various vegetative traits represent the same, or different, developmental processes. miR156 has been shown to be the master regulator of vegetative development in plants. Overexpression of miR156 prolongs the juvenile phase of development, whereas knocking-down the level of miR156 promotes the adult phase of development. Therefore, artificial modulation of miR156 expression is expected to cause corresponding changes in vegetative-specific traits in different plant species, particularly in those showing no substantial difference in morphology during vegetative development. To identify specific traits associated with the juvenile-to-adult transition in tobacco, we examined the phenotype of transgenic tobacco plants with elevated or reduced levels of miR156. We found that leaf shape, the density of abaxial trichomes, the number of leaf veins, the number of stomata, the size and density of epidermal cells, patterns of epidermal cell staining, the content of chlorophyll and the rate of photosynthesis, are all affected by miR156. These newly identified miR156-regulated traits therefore can be used to distinguish between juvenile and adult phases of development in tobacco, and provide a starting point for future studies of vegetative phase change in the family Solanaceae.