The life history of mammals underlies a fast–slow continuum, ranging from “slow” species with large body size, delayed sexual maturation, low fertility, and long lifespan, to “fast” species showing the opposite traits. Primates fall into the “slow” category, considering their relatively low offspring numbers and delayed juvenile development. However, social and sexual maturation processes do not necessarily have to be completed simultaneously. The comparison of the timeframes for sexual and social maturation is largely lacking for primates, with the prominent exception of humans. Here, we compare both maturation processes in a basal primate, the gray mouse lemur, which ranges in many aspects at the fast end of the slow–fast life history continuum among primates. We compared the patterns and frequencies of various social and solitary behaviors in young adults (YA, 12–13 months old) and older individuals (A, ≥2 years) of both sexes outside estrus. Observations were conducted during mix‐sexed dyadic encounter experiments under controlled captive conditions (eight dyads per age class). Results indicate that although all young adults were sexually mature, social maturation was not yet completed in all behavioral domains: Age‐dependent differences were found in the number of playing dyads, female marking behavior, female aggression, and social tolerance. Thus, this study provides a first indication that social maturation lags behind sexual maturation in an ancestral nocturnal primate model, indicating that these two developmental schemes may have been decoupled early and throughout the primate lineage. Am. J. Primatol. 77:1049–1059, 2015. © 2015 Wiley Periodicals, Inc.