The objective of this work was to produce bio-composites based on vulcanized rubber reinforced by natural fibers. To this end, coir fibers were added to vulcanized polyisoprene (PI) to determine the effect of fiber content (0, 20, 30 and 40 wt.%) and alkali surface treatment. Furthermore, interfacial adhesion was improved by using maleic anhydride grafted PI as a coupling agent. From the samples produced, a complete set of morphological (scanning electron microscopy) and mechanical (shear and tension) characterization was performed before and after natural aging. The results showed that the tensile modulus increased with fiber content for the range of conditions studied and the presence of a coupling agent always gave higher values due to better interfacial stress transfer. However, important elasticity loss (strain at break) was observed due to the low fiber elasticity. Finally, different physical aging mechanisms led to changes in the molecular configuration and a reduction of free volume caused by a post-curing phenomenon of the polyisoprene matrix and low environmental stability of the fibers. These phenomena were shown to increase the Young’s modulus of the neat matrix by eight times compared to unaged neat polyisoprene, but a smaller variation (five times) was observed for the bio-composites which was related to the environmental degradation of the coir fibers.