Background <p>The oral microbiome evolves across the lifespan, with alterations influenced by both general and oral health conditions. In older adults, frailty and oral hypofunction are common and may contribute to these microbial changes. This systematic review evaluated changes in the oral microbiota of older adults, frail individuals, and those with oral frailty or hypofunction (e.g., fewer teeth), and explored how these conditions relate to one another.</p> Methods <p>A comprehensive literature search was conducted across MEDLINE (PubMed), Web of Science, and the Cochrane Library. The last search was performed on 16 November 2025. Studies reporting the effects of aging, frailty, or oral frailty/hypofunction on oral bacteria or microbiome were included. All eligible studies were descriptively analyzed. Risk of bias was assessed using the Newcastle–Ottawa Scale.</p> Results <p>Thirty-four studies were included, of which 16 used conventional microbiological methods and 18 used 16S rRNA sequencing. Findings on microbial diversity (alpha and beta) associated with aging were inconsistent; however, significant compositional changes were observed in frailty. At the phylum level, Bacillota decreased and Bacteroidota increased, and at the genus level, <i>Streptococcus, Veillonella, </i>and<i> Haemophilus</i> decreased, whereas <i>Prevotella</i> increased in older adults compared with younger adults. These patterns were reversed in frail individuals and older adults with edentulism or fewer teeth compared with healthy older adults.</p> Conclusions <p>Frailty and having fewer teeth appear to exert a stronger influence on the oral microbiome than healthy aging. While older adults may develop a microbial community distinct from that of younger adults, frailty is associated with a microbial imbalance, demonstrated by a reduction in taxa important for biofilm formation and an increase in pathogenic taxa. Similar patterns are observed in individuals with fewer teeth, though evidence linking other aspects of oral hypofunction remains limited.</p> Clinical significance <p>Oral microbiome alterations may be associated with frailty. However, current evidence is not yet sufficient to support their use as diagnostic biomarkers. Further longitudinal studies are needed to clarify whether these associations are correlational or causal, which is essential for future biomarker development and microbiome-targeted interventions.</p> PROSPERO registration <p>CRD42024628795.</p>