This study was conducted to compare the aggressive behaviors of commercially housed pigs when mixed at different times after weaning. The behavioral patterns of 36 groups of pigs (a total of 360 animals) were observed by continuous monitoring over 3 consecutive days directly after weaning (25 ± 1.2 d of age), and 50 and 75 d with the aid of video technology. The pigs were not mixed previously and were same ages. The data were analyzed by ANOVA, Tukey’s post hoc tests, and Wilcoxon rank-sum test using the SAS software. Fight latency and total duration and frequency of fighting were significantly different among the age groups. The agonistic behaviors decreased in 75-d old pigs if compared with 25- and 50-d-old animals. Moreover, dominance index (DI, DI was defined as the sum of wins minus defeats divided by the sum of wins, defeats, and stand-off outcomes) was higher in 25-d-old and lower in 75-d-old pigs. A comparison of dominant (DI >0) and submissive (DI <0) pigs showed significant differences (P < 0.05) for major aggressive behaviors in all age groups. Higher frequency and longer duration of aggressive interactions mean that dominant animals were more active than the submissive ones. Moreover, pigs with a positive dominance index initiated more fights (P < 0.05). Dominant pigs were involved in more aggressive interactions, had longer fights, and initiated more fights than submissive pigs. Moreover, older pigs have shorter fights after mixing and that they also sustain fewer injuries from these fights. Post-mixing aggressive behavior changed over time. Early experience of mixing in the rearing conditions might contribute to reduced aggressive behavior of growing pigs. It may have been occurred by more energetically efficiency strategy for stable social structure. This finding has potentially important consequences for animal welfare and economic production of commercial swine production.