With tetras it depends on the species but I think most get to around 5 though I think many live for double that.
Platies, BTW, can live to 4 years sometimes and most bettas don't realy get to 4 years of age though they should.
As for the cories, I'm not realy shure but I think it's fair to say that they live a considerably long time - catfish tend to be long-lived when compared to other species.