<p><i>Neoseiulus californicus</i> (McGregor) is one of the main predators belonging to the family Phytoseiidae classified as a type II generalist predator that has been used to control greenhouse pests such as thrips, eriophyids, and spider mites. One of the significant ways to recognize the efficacy of the phytoseiid predatory mites is the study of their foraging behavior. Mutual interference is a negative relationship between a predator’s searching efficiency and its density. This study appraised the effect of the age-dependent mutual interference of <i>N. californicus</i> on larvae and nymphs of <i>Tetranychus urticae</i> Koch under laboratory conditions at 25 ± 1℃, 70 ± 5% RH and a photoperiod of 16&#xa0;L: 8D h. To determine the per capita searching efficiency and interference coefficient, Nicholson’s model and linear regression were used, respectively. The results showed that the total predation of <i>N. californicus</i> reared on <i>T. urticae</i> nymphs was significantly affected by predator density and age. In addition, per capita searching efficiency of the predator significantly declined with predator density, which is the result of mutual interference. Furthermore, the slope of the linear regression (<i>m</i>) between the logarithm of per capita searching efficiency and the logarithm of the predator density increased, ranging from − 0.465 in 5-day-old individuals to -0.1452 in 40-day-old individuals. The negative effects of mutual interference decreased along with growing age. Our findings showed that in different tested ages, the rate of interference among adults differed compared to immature individuals. These results suggest that age-specific differences in mutual interference should be considered when optimizing the mass rearing and field release of predators for the biological control of <i>T. urticae</i>.</p>