16S rRNA gene sequencing data plotted from months 3–40 of life (n = 11,717). Curves show LOESS fit for the data per category, and shaded areas show permutation-based 95% confidence intervals for the fit. Significance determined by linear mixed-effects models in accordance with observed phases of maturation: developmental (months 3–14), transitional (months 15–30), and stable (months 31–46). Shaded lines represent the ±95% confidence interval. Longitudinal development of the Shannon’s diversity index, microbiota age and MAZ score by geographical location (a–c), occurrence of household siblings (d–f), and occurrence of household furry pets (g–i).