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Okay so a bunch of questions....

How long does it take for a peninsula cooter to grow to a decent size ? Decent like 8inches or more...
Can 3inch cooters be sexed? Are these animals as hardy as a red eared?

A bit late but I guess their growth is ties to how much they are fed. I would think 4-5 years it could reach 8", even faster depending on diet. 3" is kinda small. You might be able to sex earlier if the male characteristics popup before the 4" mark. Yes, they are similar to RES.