The rumen plays an essential role in the physiology and health of ruminants. The rumen undergoes substantial changes in size and function from birth to adulthood. The cellular and molecular mechanisms underlying these changes are not clear. This study aimed to identify the transcription factors (TFs) and signaling pathways mediating these changes in cattle. We found that the ratios of the emptied rumen, reticulum, omasum, and abomasum to body weight in adult steers were 4.8 (P < 0.01), 3.1 (P < 0.01), 6.0 (P < 0.01), and 0.8 (P = 0.9) times those in neonatal calves, respectively. The length of rumen papillae and the thickness of rumen epithelium, tunica mucosa and submucosa, tunica muscularis, and tunica serosa increased 7.4-, 2.0-, 3.0-, 2.9-, and 4.6-fold (P < 0.01 for all), respectively, from neonatal calves to adult steers. However, the density of rumen papillae was lower in adult steers than in neonatal calves (P < 0.05). The size of rumen epithelial cells was not different between neonatal calves and adult steers (P = 0.57). RNA sequencing identified 2,922 genes differentially expressed in the rumen between neonatal calves and adult steers. Functional enrichment analyses revealed that organ development, blood vessel development, Ras signaling, and Wnt signaling were among the functional terms enriched in genes downregulated in adult steers vs. neonatal calves and that fatty acid metabolism, immune responses, PPAR signaling, and Rap1 signaling were among those enriched in genes upregulated in adult steers vs. neonatal calves. Serum response factor (SRF), interferon regulatory factor 4, and purine-rich single-stranded DNA-binding protein alpha were among the major candidate TFs controlling the expression of genes upregulated, while TCF4, inhibitor of DNA binding 4, and snail family transcriptional repressor 2 were among those controlling the expression of genes downregulated in adult steers vs. neonatal calves. Taken together, these results suggest that the rumen grows by increasing the number, not the size, of cells from birth to adulthood, that the absorptive, metabolic, immune, and motility functions of the rumen are acquired or significantly enhanced during the postnatal life, and that the changes in rumen size and function from birth to adulthood are mediated by many candidate TFs, including SRF and TCF4, and many candidate signaling pathways, including the PPAR and Wnt signaling pathways.