Rank importance of OTUs determined by applying the random forest regression to the chronological age of 150 full-term, vaginally delivered, breastfed infants (n = 2,871 stool samples). The importance of OTUs is determined by the percentage increase in mean-squared error of microbiota age prediction when the relative abundance of each OTU were randomly permuted (mean importance ± s.d., n = 100 replicates). These selected OTUs explained 72% of the variance (compared to 75% variance explained with all OTUs in model) and were used to define maturation of the gut microbiome by microbiota age and MAZ score. OTUs are named to the genus level and coloured based on association with life stage; blue were associated with samples collected in the first 15 months, green with samples collected between months 15 and 30, and red were with samples collected after month 30. a, Twenty OTUs ranked by importance to the accuracy of the model. The tenfold cross-validation error is also displayed in order of variable importance. Blue dotted line represents the 20 OTUs used in the model. b, Heat map of mean relative abundance of the 20 selected OTUs per month from 3 to 40 months of age.