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Introduction While exertional breathlessness increases with age, its underlying mechanisms remain poorly understood. In many physiological systems, ageing leads to less variable and more constrained patterns. We hypothesized that ageing leads to a more constrained ventilatory pattern during exercise. We utilized non-linear statistical modelling (Sample Entropy – SampEn) to characterize the ventilatory response to exercise in a study of healthy ageing.Methods We analysed data from participants in the COmPLETE study who underwent an incremental cardiopulmonary exercise test to exhaustion. SampEn for minute ventilation (VE), breathing frequency (Bf) and tidal volume (VT) were calculated. The relationship between ventilatory entropy parameters and age was assessed using multiple linear regression models with penalized cubic splines for age, adjusting for sex, BMI, peak oxygen uptake, and FEV1.Results A total of 526 healthy adults (mean age 54 [range 20–92]; 52% female) were included in the analysis.Contrary to our hypothesis, the relationship between age and ventilatory complexity was significantly non-linear (p < 0.001 for VE). The trajectory showed that the SampEn VE remained stable through young adulthood and middle age, before increasing progressively from approximately 60 years onwards.This pattern of increasing complexity in later life was consistent for both breathing frequency (Bf) (p=0.023) and tidal volume (VT) (p=0.004), indicating the ventilatory pattern becomes more irregular with advanced age.Conclusions In healthy ageing, the ventilatory pattern during exercise does not become more constrained but rather significantly more complex after the sixth decade of life. Therefore, future studies should focus on validating Sample Entropy as a tool to better assess breathing pattern irregularity.Abstract S25 Figure 1