This chapter critiques the expansion of the original nine Hallmarks of Aging and argues that most of these are downstream consequences of mitochondrial dysfunction and the resulting ATP energy deficit, which is proposed as the primary driver of human aging. The chapter surveys current methods for estimating biological age—including fitness and cognitive testing, blood‑based biomarkers, telomere length assays, epigenetic clocks, protein‑expression noise, senescent‑cell burden, mitochondrial DNA copy number, and emerging direct mtDNA damage assays—and highlights the limitations of relying on easily sampled tissues such as blood and saliva. Drawing on recent technologies such as ddPCR (to measure mtDNA number) and MitoClock assays (to directly identify mtDNA point and deletion mutations), it emphasizes the tissue‑specific nature of mitochondrial damage and the need for more accurate, organ‑level measures of aging. Overall, the chapter argues for a unified, energy‑centric framework for understanding aging and for more rigorous, quantitative tools to evaluate longevity interventions.