A mother's gift

During and after childbirth, bacteria from the mother’s gut take up residence in the baby’s body, seeding a unique community of beneficial bacteria that will help break down food, synthesize vitamins, and help teach the baby’s nascent immune system to recognize foreign organisms.
According to a new study Scientists at the Broad Institute of MIT and Harvard discovered that during pregnancy and the first year of life, some bacteria in the mother’s gut donate hundreds of genes to bacteria in the baby’s gut. 
Also, the baby's GM is impacted by maternal medications, stress, vaccinations, chemical exposures, diet, use of antibiotics, infection, host genetics., and perinatal exposures such as delivery mode, and perinatal medications [1] family lifestyle, geographic location.

Transitioning Milk to Solids

By the age of 36 months, the infant's gut microbiota develops into an adult-like gut microbiota, which is dominated by Firmicutes and Bacteroidetes phyla (this makeup>90% of the gut microbial population)[3]During the first few months of life, the primary source of nutrition is breast milk or formula. Therefore, the infant intestine at this stage favors the propagation of microbes such as Bifidobacterium that can ferment milk oligosaccharides. The introduction of solid food is a major contributor to the shift of the gut microbiome toward an adult-like structure, which was previously characterized by an increase of Bacteroidetes and the Firmicutes and a decrease in Actinobacteria and Proteobacteria phyla (Arrieta et al., 2014). The rationale behind this change is that the infant diet at this stage contains many polysaccharides that cannot be digested by their immature digestive system, thereby triggering an increase of the aforementioned microbes to facilitate this process. 
3. Ringel-Kulka et al., 2013; Voreades et al., 2014.

Adulthood changes

Overall, the adult microbiome remains relatively stable except following challenges such as infections, antibiotic treatment, or dietary changes. 
Even though our microbiome has the ability to recover over time, these challenges may subtly alter our unique microbial makeup.

As we age, it’s natural to lose some of the diversity in our gut microflora. 

For instance, Bifidobacteria are abundant in the infant microbiome and remain stable in adulthood. They then start to slowly decrease as the normal lifestyle changes associated with our golden years influence the composition of our microbiomes. These changes have been linked to increased risk of several chronic age-related diseases.