Aged polyethylene microplastics dramatically altered gut bacterial communities in marine fish larvae during 30-day exposure, with UV-weathered particles enriching Roseibacillus bacteria while Fenton-aged particles increased Ruegeria and Pseudomonas populations. Both aging processes changed the particles' surface properties and toxicity profiles compared to fresh plastic fragments. The microbiome disruptions weren't merely compositional—they fundamentally rewired metabolic functions, with Fenton-aged particles upregulating metabolism-related pathways as a stress response rather than health benefit. This represents a significant advance in microplastics research, moving beyond commercial laboratory particles to simulate real-world weathering processes that occur in marine environments. The gut microbiome connection is particularly relevant for human health, as similar bacterial disruptions could theoretically occur in humans consuming microplastic-contaminated seafood or water. While this fish larvae study can't directly predict human outcomes, it reveals how environmental plastic aging creates more complex toxicity profiles than previously understood. The 30-day exposure timeline and specific bacterial strain identification provide concrete evidence that microplastic environmental persistence translates to biological persistence, potentially explaining why microplastic health concerns have intensified as ocean plastic pollution ages.