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Concrete structures, such as armories, military bunkers, and petrochemical facilities, are susceptible to explosive loads during service. The superior mechanical properties of ultrahigh-performance concrete enable structures built with it to endure explosive loads. This study aims to characterize the behavior of ultrahigh-performance concrete at different ages, namely, 1, 3, 7, 14, and 28 days, at different strain rates. The compressive stress–strain response of ultrahigh-performance concrete is studied at static and high-strain-rate loading. The high-strain-rate response is characterized using a split-Hopkinson pressure bar of 100-mm diameter. The influence of age on strength, dynamic increase factor strain rate, elastic modulus, peak strain, and energy absorption are discussed. When tested at the same striker velocity, the dynamic increase factor and strain rate of ultrahigh-performance concrete of similar specimen size decrease with age. The elastic modulus and energy absorption increase from 1 to 14 days. There is no significant change in these quantities between 14 and 28 days. The strength, elastic modulus, and energy absorption increase with increasing strain rate for all ages. Experimental data are validated using the Tedesco model and current standards for stress, strain, and elastic modulus. The model from the standards overestimates strength, whereas the Tedesco model predicts strength conservatively. [ABSTRACT FROM AUTHOR]