At the heart of the review is a critical comparison of the major frailty assessment tools available to researchers working with naturally aged mice. The mouse frailty index, first adapted from human deficit accumulation models, quantifies the proportion of health deficits an animal has accumulated across observable domains such as body condition, fur quality, tremor, kyphosis, vestibular disturbance, and gait. Each deficit is scored on a simple ordinal scale, and the scores are summed and divided by the number of items measured to yield an index between zero and one. Because the approach mirrors the clinical deficit accumulation framework introduced by Rockwood and Mitnitski, it offers a direct translational bridge: in both humans and mice, index values rise with age, correlate with mortality risk, and respond to interventions such as rapamycin. The review emphasizes that this instrument has demonstrated strong inter-rater reliability and has been validated across laboratories, but it also carries well-known caveats. Scores depend on the observer’s subjective judgment, and the interpretation of an index value can shift depending on the specific battery of items included.

The mouse frailty phenotype represents the second major family of tools, transposing the five criteria of the Fried phenotype—involuntary weight loss, exhaustion, low physical activity, slowness, and weakness—into mouse-appropriate measurements. Grip strength testing on a force transducer substitutes for handgrip dynamometry, treadmill endurance or open-field activity replaces self-reported exhaustion, and body weight trajectories are tracked longitudinally to capture weight loss. A significant strength of the phenotype approach is its objectivity, since the underlying measurements are instrument-based rather than observation-based. However, the review highlights a persistent methodological difficulty: clinical frailty phenotyping uses sex- and cohort-specific percentile cutoffs, typically the lowest twenty percent of a reference population, which means that classification depends on the statistical distribution of the cohort being studied rather than on a fixed biological threshold. This cohort dependency complicates comparisons across studies, strains, sexes, and laboratories, and it can yield divergent prevalence estimates for what may be biologically similar animals.

Beyond these two dominant frameworks, the review catalogues a growing family of composite scores that extend the dimensional coverage of frailty assessment. Physical function scores and vitality scores combine performance-based measures such as grip strength, rotarod endurance, gait speed, and body composition into weighted composites intended to capture functional reserve more continuously than categorical phenotyping. More recently, researchers have introduced domain-specific instruments that acknowledge frailty is not exclusively physical. The mouse social frailty index assesses impairments in social behavior, drawing on the growing recognition in human geriatrics that social withdrawal and reduced social participation predict adverse outcomes independently of physical decline. Similarly, a novel cognitive frailty index for geriatric mice integrates cognitive testing with physical parameters, translating the clinical concept of cognitive frailty—simultaneous physical frailty and cognitive impairment without dementia—into a rodent-appropriate framework. These newer tools remain less extensively validated than the core index and phenotype, but they signal an important expansion of the conceptual space that mouse frailty assessment is expected to cover.

The review then connects measurement to mechanism, asking which biological processes actually drive the deficits that these instruments record. Drawing on the hallmarks of aging framework, the authors examine the contributions of chronic low-grade inflammation, or inflammaging, immunosenescence, mitochondrial dysfunction, cellular senescence, altered nutrient sensing, and impaired stress resilience. The evidence linking interleukin-6 to frailty is singled out as particularly striking: experimental work has shown that elevated IL-6 can, by itself, drive many features of the frailty phenotype in mice, and inducible humanized IL-6 knock-in models have demonstrated that this cytokine accelerates physical decline partly through mitochondrial dysregulation. Senolytic interventions that clear senescent cells have been shown to improve physical function and extend lifespan in aged mice, while metabolomic studies have identified signatures of energy metabolism—including vitamin E and carnitine shuttle mechanisms—associated with frailty in both mice and humans. Mitochondrial dysfunction is emerging as a candidate biomarker of frailty, with ongoing studies probing its diagnostic potential. These mechanistic threads suggest that the deficits scored in a frailty index are not arbitrary but reflect converging failure of the immune, metabolic, and mitochondrial systems.

Despite this mechanistic coherence, the review is unsparing in its assessment of the limitations of current instruments. Subjective observation remains embedded in the deficit-based index, and even instrument-based phenotype measurements are sensitive to testing conditions: time of day, handling history, ambient temperature, housing density, apparatus design, and prior exposure to testing can all influence performance. The review invokes classic multi-laboratory studies demonstrating that mouse behavior is profoundly shaped by laboratory environment, a warning that resonates strongly in the frailty field, where behavioral tests such as maze tasks and grip strength assays are common components. Validation across sex, strain, laboratory, and outcome remains uneven. Most frailty indices were developed in C57BL/6J males, and while sex-specific analyses have revealed that frailty components differ between males and females, systematic validation in female cohorts, in genetically heterogeneous stocks, and in diversity outbred populations is still incomplete. Cohort-specific thresholds, the review argues, should be treated as a structural limitation rather than an incidental nuisance, because they affect how prevalence estimates and intervention effects should be interpreted.

Looking toward the future, the review identifies several promising but still exploratory directions. Digital phenotyping stands out as the most transformative. Machine-vision-based frailty indices that extract postural and movement features from video recordings of mice in their home cages promise to remove observer subjectivity entirely, and automated home-cage monitoring systems have already been shown to detect age-associated morbidity in both inbred and genetically diverse mice. Machine learning approaches have also been used to construct age and life expectancy clocks from frailty data, demonstrating that the information content of a frailty assessment extends beyond a single score. Longitudinal trajectory modeling, including joint models of repeated measurements and mortality, may capture the dynamics of deficit accumulation in ways that cross-sectional snapshots cannot. The review also discusses secondary prognostic models and measures of latent vulnerability, including physiological resilience—the capacity to recover from acute stressors—as concepts that could enrich frailty assessment. However, the authors are careful to draw a firm line: alternative weighting systems and the proposed distinction between pre-frailty and subclinical frailty require prospective validation before they can be considered standardized methods, and researchers should not adopt them as established practice.

The central message of the review is one of methodological discipline. Naturally aged mice remain an indispensable platform for frailty research precisely because the syndrome emerges from genuine biological aging, and the available instruments capture complementary aspects of the phenotype. Yet the field’s translational promise depends on clear separation of what has been validated from what remains conceptual. By systematically comparing the mouse frailty index, mouse frailty phenotype, physical and vitality scores, social and cognitive frailty indices, and emerging digital approaches against their clinical counterparts, the authors provide a roadmap for rigor: choose the instrument appropriate to the research question, report scoring procedures transparently, acknowledge cohort- and sex-specific limitations, and resist premature standardization of unvalidated extensions. As preclinical frailty research continues to inform the design of human anti-aging interventions, that rigor may determine whether laboratory findings survive the journey to the clinic.