Early-life gut microbiome development is a dynamic process with profound implications for child health, yet its rapid changes during infancy pose challenges for its integration into pediatric models. In this study, we present a microbiome age model that captures typical gut microbial maturation trajectories from 2 to 18 months of age. Built from over 3,100 infant gut metagenomes spanning 12 countries and a range of socioeconomic contexts, our model uses taxonomic profiles to predict microbial age with high accuracy (cross-validation RMSE: 2.56 months; R²: 0.64). We found that only about 20% of the input features drive the majority of predictive performance, with consistent patterns across different geographical sites. These include a decline in early-dominant taxa, such as Bifidobacterium spp., and an increase in later colonizers, such as Faecalibacterium prausnitzii, reflecting predictable ecological succession during the weaning period. Functional annotation of age-associated genes revealed shifts in metabolic potential, particularly related to carbohydrate metabolism, aligned with dietary transitions. This work provides a generalizable reference model for estimating age in infancy based on the gut microbiome. By establishing a reference trajectory for gut microbiome maturation, this model supports early identification of atypical – hastened or delayed – developmental patterns, offering potential for integration into multi-omic pediatric health frameworks. We are currently applying this model in broader contexts, including efforts to predict neurodevelopmental outcomes across diverse global populations.