Background: Circulating Osteoglycin (OGN), a proteoglycan is released from both bone and muscle and has been associated with markers of metabolic health. However, it is not yet clear whether the levels of circulating OGN change throughout the adult lifespan or if circulating levels of OGN are associated with metabolic markers or aerobic fitness levels. Methods: 107 individuals (46 males and 61 females) aged 21-87 years were included in the study. Serum OGN levels, aerobic capacity (VO2peak), glucose and homeostatic model assessment for insulin resistance (HOMA-IR) were assessed. T-tests were used to compare participant characteristics between sexes. Regression analyses were performed to assess the relationship of OGN with age, aerobic fitness and metabolic markers. Results: OGN displayed a “U shaped” relationship with age in both males and females. However, males had higher absolute levels of OGN than females across the lifespan (β=0.23, p=0.03). Overall, age and sex explained 16 % of the variance in OGN (adjusted R2 = 0.16; p < 0.001). Higher OGN was associated with higher VO2peak (β = 0.02, p = 0.001) however those aged below 50 showed a stronger positive relationship than those aged above 50. A higher OGN level was associated with a higher circulating glucose level (β = 0.17, p < 0.01). No association was observed between OGN and HOMA-IR. Conclusions: OGN across the lifespan was characterized by a U-shaped curve and this was similar between sexes. Those with a higher aerobic capacity or higher baseline glucose concentration had higher OGN levels. Our data suggests an association between OGN and aerobic fitness and glucose regulation. Whether it plays a direct mediating role in either aerobic fitness or glucose regulation humans is still unknown.