Mitochondria integrate bioenergetics, redox signalling, calcium handling, biosynthesis, apoptosis, and stress responses. Their contribution to ageing depends less on any single pathway than on the ability to sustain these functions through continuous maintenance, remodelling, and inter-organelle communication. This review proposes mitochondrial homeodynamics as a systems-level framework for that ability, which rests not on static preservation but on three linked capacities. Maintenance safeguards mitochondrial genome, proteome, and membrane integrity. Adaptation adjusts metabolism and remodels network and cristae architecture to match changing demand. Recovery restores function and reserve after challenge. These capacities emerge from mitochondrial quality control, network and cristae remodelling, biogenesis, mitophagy, retrograde stress signalling, and inter-organelle communication. So defined, mitochondrial dysfunction becomes a measurable loss of capacity rather than a descriptive category. Ageing erodes these capacities in tissue- and context-specific ways, which reduces physiological reserve, slows recovery after stress, and amplifies sterile inflammation. The mechanisms underlying these capacities, the biomarkers that report them, and the interventions proposed to preserve them are evaluated in turn. Exercise provides the strongest human evidence for coordinated mitochondrial and functional adaptation, whereas evidence for energy restriction, NAD+ precursors, mitophagy-supporting compounds, and mitochondria-targeted agents remains heterogeneous and endpoint-specific. No mitochondrial intervention has been shown to slow ageing or extend lifespan in healthy humans, and movement of a biomarker towards a younger reference value does not establish rejuvenation. Progress will require dynamic measures of maintenance, adaptation, and recovery, obtained in defined tissues and interpreted alongside clinically meaningful outcomes.
