Appreciation of the importance of the gut microbiome is growing, and it is becoming increasingly relevant to identify preventive or therapeutic solutions targeting it. The composition and function of the gut microbiota are relatively well described for infants (less than 3 years) and adults, but have been largely overlooked in pre-school (3–6 years) and primary school-age (6–12 years) children, as well as teenagers (12–18 years). Early reports suggested that the infant microbiota would attain an adult-like structure at the age of 3 years, but recent studies have suggested that microbiota development may take longer. This development time is of key importance because there is evidence to suggest that deviations in this development may have consequences in later life. In this review, we provide an overview of current knowledge concerning the gut microbiota, its evolution, variation, and response to dietary challenges during the first decade of life with a focus on healthy pre-school and primary school-age children (up to 12 years) from various populations around the globe. This knowledge should facilitate the identification of diet-based approaches targeting individuals of this age group, to promote the development of a healthy microbiota in later life. The gut microbiota of healthy children displays functional and taxonomic differences with respect to those of adults, suggesting that the gut microbiome may develop more slowly than previously thought.Bifidobacterium spp. are more abundant in the gut microbiota of children than in that of adults, and may gradually decrease until adulthood.The microbiota may develop more slowly in some children than in others, who may present an intermediate microbiota state.Childhood may provide additional opportunities for microbiota-based interventions to promote health or prevent microbiota deviation.The gut microbiota of children may be more malleable to environmental factors than that of adults.Differences in lifestyle, and westernization in particular, strongly influence the composition of gut microbial populations in children, as already reported for adults.