<p>The ageing process is often portrayed as a steady linear decline, as reflected by the methodological frameworks used to study it. Although linear models successfully capture relevant age-related mechanisms, they may fail to identify key transition states occurring during the lifespan. Indeed, ageing entails a series of transitions — from early development, through adolescence, to older age — that are each characterized by distinct biological remodelling events. The study of these underlying patterns, through an appropriate framework, is essential because interventions may only be effective during specific windows when the system transitions to a new state. This Perspective highlights the evidence for non-linearity in ageing, overviews current analytical approaches to capture non-linear processes and then discusses important considerations for ageing research going forward.</p>