The aim of this paper is to analyze the effect of water aging of flax/PLA composites on their mechanical characteristics and damage mechanisms. The composites were manufactured with three-dimensional (3D) printer using bio-based material which is polylactic acid reinforced with flax fiber. The composites studied were subjected to water immersion at room temperature and tested in static and fatigue tensile tests for different immersion times. During mechanical tests, the acoustic emissions (AE) method was utilized to identify the emergence of damage processes and to track their evolution. Moreover, macroscopic and microscopic data were used to study damage processes. To confirm the reality of the damage mechanisms observed, microscopic data were linked with acoustic emission classes. Two types of events were observed by acoustic emissions associated with two composite damage mechanisms. Young’s modulus and stress decreased with aging, according to static testing. However, there was an increase in the strain. Strength and damping properties of unaged and aged composites were investigated using fatigue tensile tests. The results showed that water uptake induced a decrease in strength and stiffness and an increase in damping when compared in terms of aging time. There was also a reduction in useful life as immersion time increased.