Background: The 30-s sit-to-stand (STS) muscle power test is a valid test to assess muscle power in older people; however, whether it may be used to assess trajectories of lower-limb muscle power through the adult lifespan is not known. This study evaluated the pattern and time course of variations in relative, allometric and specific STS muscle power throughout the lifespan. Methods: Subjects participating in the Copenhagen Sarcopenia Study (729 women and 576 men; aged 20 to 93 years) were included. Lower-limb muscle power was assessed with the 30-s version of the STS muscle power test. Allometric, relative and specific STS power were calculated as absolute STS power normalized to height squared, body mass and leg lean mass as assessed by DXA, respectively. Results: Relative STS muscle power tended to increase in women (0.08 ± 0.05 W·kg⁻¹·yr⁻¹; p = 0.082) and increased in men (0.14 ± 0.07 W·kg⁻¹·yr⁻¹; p = 0.046) between 20 and 30 years, followed by a slow decline (−0.05 ± 0.05 W·kg⁻¹·yr⁻¹ and −0.06 ± 0.08 W·kg⁻¹·yr⁻¹, respectively; both p > 0.05) between 30 and 50 years. Then, relative STS power declined at an accelerated rate up to oldest age in men (−0.09 ± 0.02 W·kg⁻¹·yr⁻¹) and in women until the age of 75 (−0.09 ± 0.01 W·kg⁻¹·yr⁻¹) (both p < 0.001). A lower rate of decline was observed in women aged 75 and older (−0.04 ± 0.02 W·kg⁻¹·yr⁻¹; p = 0.039). Similar age-related patterns were noted for allometric and specific STS power. Conclusions: The STS muscle power test appears to provide a feasible and inexpensive tool to monitor cross-sectional trajectories of muscle power throughout the lifespan.