1. Abstract

Biological aging is complex and resists any single mechanistic narrative. This article develops a conceptual framework for aging biology through a series of brief statements on central topics in the field, with the goal of discussing aging simply, directly and accessibly. The organizing principle is that aging is a default outcome of living: because biological processes are inherently imperfect, deleterious changes (the deleteriome) accumulate over time and progressively erode functional reserve. The deleteriome provides a unifying framework for molecular damage, regulatory drift, maladaptive remodeling, and system-level failure, without reducing aging to individual genes, pathways, compartments, hallmarks, or pathologies. Evolution does not program aging; it shapes maintenance and renewal systems that oppose this trajectory with limited fidelity and unavoidable trade-offs. Aging begins during development, following an early embryonic phase of rejuvenation. Modern biomarkers make biological age imperfectly but increasingly measurable. Biological age is heterogeneous across cells, organs, and systems, modifiable by interventions and environment, and in some contexts partially reversible or resettable, although aging itself is irreversible. This model links first principles of aging to measurement, intervention, and longer-term strategies, with the goal of clarifying how substantial longevity gains might be achieved, both within and beyond existing species-level constraints.