Understanding the extent to which genetic variation underlies behavioral variation is key to understanding how behavior might evolve. We investigated whether genetic variance contributes to variation in aggressive behavior among individuals in mangrove rivulus fish. We also examined whether broad-sense heritability for aggression changes over a life-history transition from juvenile to adult. Rivulus provides a unique opportunity to investigate the relative contributions of genetic and environmental variance to phenotypic variation because they exist primarily as self-fertilizing hermaphrodites. This mating strategy permits natural production of nearly isogenic lineages and analysis of genetic effects on the phenotype independent of environmental effects without complex breeding designs. We hypothesized that there would be: i) age-dependent variation among genotypes in aggressive behavior and ii) aggressive behavior would differ between juveniles and adults. We quantified aggression of 8 genotypes of rivulus (N=64) using a 3-D printed model opponent and allowed individuals to interact with the model for 30 minutes. We tested animals four times throughout the experiment, twice as juveniles (80 and 87 days post-hatch (dph)) and twice as adults (173 and 180 dph). We found that juveniles were more aggressive than adults. We also found that aggressive behavior was repeatable in both juveniles and adults but heritable only in juveniles. Additionally, we found significant variation among genotypes (i.e, heritability) in the extent to which aggression changed over development. Although we found individual variation for aggressive behavior in both life stages, the data suggest that aggressive behavior will evolve at a higher rate in juveniles than adults when exposed to the same selection pressures.