Hypothesis: With advancing age, a decline in butyrate-producing gut microbes reduces luminal short-chain fatty acid (SCFA) concentrations, diminishing HDAC inhibition in enterochromaffin (EC) cells 2. This loss of epigenetic restraint elevates TPH1 transcription and pushes EC cells into a senescent state marked by p16INK4a upregulation, γH2AX foci, and a senescence-associated secretory phenotype (SASP) that includes excess serotonin, IL-1β, ATP, and chemokines such as CXCL8. Senescent EC cells exhibit impaired Piezo2-mediated mechanotransduction 4, leading to reduced gut motility and aberrant release of signaling molecules that activate vagal afferents via 5-HT3 receptors and purinergic channels. The resulting chronic vagal drive primes central microglia, amplifying systemic inflammaging markers such as IL-6 and CRP and accelerating cognitive decline. Dysregulated EC-derived 5-HT has been implicated in gut motility disorders and neurodegenerative diseases where α-synuclein colocalizes in EC cells 3, supporting the idea that EC senescence could propagate pathology via gut-brain connections. Furthermore, EC cells function as polymodal chemosensors that couple microbial metabolites, mechanical stimuli, and dietary factors to both enteric and central nervous system signaling 1, positioning them as a critical hub where microbiome aging translates into neuronal dysfunction. Thus, EC cell senescence functions as a mechanistic link between microbiota aging, gut dysfunction, and neuroinflammation. Predictions: (1) Older individuals with low fecal butyrate will show higher frequencies of p16INK4a+/γH2AX+ EC cells in colonic biopsies compared to those with high butyrate; (2) The density of senescent EC cells will positively correlate with plasma IL-6, CRP, and inversely with cognitive test scores; (3) Experimental restoration of luminal butyrate in aged mice will reduce EC senescence markers, normalize serotonin release, improve motility, and attenuate microglial activation; (4) Pharmacological clearance of p16+ EC cells (e.g., via senolytics) will break the correlation between low butyrate and inflammaging, confirming causality. Falsification: If EC senescence markers do not rise with age-independent of butyrate levels, or if senescent EC depletion fails to modify systemic inflammation despite corrected microbiota, the hypothesis would be refuted.