I think most can be sexed at a month, but I think growth with fry depends a lot on what they are/were fed, water quality, and genetics. Some people can sex their fry when they are just a few weeks old, but other times it may take a few months. I would say the safest way to sex guppies is to look at the anal fin. Males have the skinny pointy ones, and the ladies have little fans. Do keep in mind, though, that some babies may mature later than others. I had what I thought was a girl balloon molly turn out to be a male for me. He was just a late bloomer
