The production of bio-composites based on vulcanized rubber reinforced by natural fibers has been widely studied. However, the aging phenomenon occurring during the long-term storage generating irreversible damage is limiting the performance of this kind of composites which has not been studied in details. So the objective of this work was to characterize and understand the structure-mechanical property relationships in these bin-composites and to correlate the effective crosslinking density with physical aging as a function of fiber content (0, 20, 30 and 40 wt.%) and fiber surface modification (alkali treatment) for the system polyisoprene (PI) and coir fibers. Furthermore, interfacial adhesion was improved by using maleic anhydride grafted polyisoprene 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 tensile strength 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 the unaged neat polyisoprene, but a smaller variation (five times) was observed for the bio-composites which was related to different crosslinking density and coir fibers stale of dispersion in the vulcanized matrix.