Markers of oxidative stress and biochemical composition were investigated in digestive gland and gonad tissues in the freshwater mussel Diplodon chilensis, as well as morphometric variables in relation to chronological age and sex. Individual growth followed a von Bertalanffy growth model (VBGM). Superoxide dismutase activity, glutathione level and oxidative damage to proteins remain constant through the life of both tissues, whereas catalase and glutathione-S-transferase activities and lipid peroxidation decrease until 24–27 years of age, to remain fairly stable (mostly in the gonads) or increase slowly (mostly in the digestive gland) afterwards. The timing of these age-related changes is coincident with the age estimated (28 years) from the lower confidence interval for L8 (the asymptotic length, 69.97 mm), at which the bivalves would reach their minimum growth rate. D. chilensis qualifies as an environmental mitigator for water and sediment clearance. Individuals near the age of minimum growth (20–30 years) would be better suited for bioremediation strategies compared with younger individuals (more sensitive) or to older ones, which are less active and show increasing lipid peroxidation with age. Utilising D. chilensis of this age class in sewage-polluted lake shores, in parallel with efforts to improve sewage treatment plants, would ensure an enhancement of the water and sediment cleansing for several decades.