Advancing age is accompanied by physiological, metabolic, and cognitive changes that are closely linked to alterations in the gut microbiome. Recent evidence demonstrates that dietary phytonutrients, prebiotics, probiotics, and postbiotics can synergistically promote active aging by modulating gut microbial composition, enhancing immune regulation, and supporting neurocognitive function. This review synthesizes recent mechanistic insights into how these bioactive compounds interact with the gut-microbiota-immune-metabolic axis to sustain health and longevity. Phytonutrients such as polyphenols and carotenoids promote microbial diversity and short-chain fatty acid (SCFA) production, reducing oxidative stress and systemic inflammation. Prebiotics, including inulin and galactooligosaccharides, selectively stimulate beneficial taxa like Bifidobacterium and Lactobacillus, improving intestinal integrity and metabolic resilience. Probiotics reinforce mucosal defense, modulate cytokine networks, and contribute to mental well-being via the gut-brain axis, while postbiotics - non-viable microbial metabolites - offer safe and potent immunomodulatory and anti-inflammatory effects. Collectively, these biotic agents form a microbiome-centered nutritional strategy capable of mitigating age-related metabolic decline, cognitive impairment, and chronic inflammation. Integrative approaches combining synbiotics and phytonutrient-rich diets represent a promising avenue for extending healthspan and fostering active, independent living in older adults.