Forty healthy participants (10 young males 23±3yrs; 10 young females 23±3yrs; 10 older males 63±3yrs, 10 older females 63±6yrs) mimicked their exercise breathing patterns in the absence of exercise across a range of exercise intensities. At peak exercise, VO2RM represented a significantly greater fraction of peak oxygen consumption (VO2peak) in young females, 12.8±3.9%, compared to young males, 10.7±3.0% (P=0.027), while VO2RM represented 13.5±2.3% of VO2peak in older females and 13.2±3.3% in older males. At relative ventilations, there was a main effect of age, with older males consuming a significantly greater fraction of VO2RM (6.6%±1.9)than younger males (4.4%±1.3;P=0.012), and older females consuming a significantly greater fraction of VO2RM (6.9%±2.5)than younger females (5.1%±1.4;P=0.004) at 65% max. Furthermore, both younger and older males had significantly better respiratory muscle efficiency than their female counterparts at peak exercise (P=0.011;P=0.015). Similarly younger participants were significantly more efficient than older participants (6.5%±1.5% vs. 5.5±2.0%;P=0.001). Age-related changes in respiratory function, and sex-based differences in airway anatomy, influence the cost to breathe during exercise. It is possible the higher fraction of VO2RM during peak exercise predispose young females and older individuals to divert more blood flow to respiratory muscles at the expense of other muscles.