It's not age that counts when sexing torts, it's size. They all look female until they grow to near full size. Before that, the only way to tell for certain is if you get 'flashed' by a maturing male.
A male tort's privates are very large and once seen never forgotten!
I'd wait till they are 5 to 6 inches to help better determine sex....note this is not 100% full proof. Like joesmum said...your so called female Georgina might show you his George if you know what I mean.