I haven't had much experience with American Green tree frogs, but I believe 8-10 years is their range. I know some of the larger species like White's and Waxy Monkey tree frogs can live about 20 years. The reason frogs live longer than a similarly sized mammal like a mouse is because frogs have slower metabolisms due to the fact that they are ectothermic (cold-blooded). Lower body temperature equals longer lifespan.