Thank Mando My understanding is growth slows down after they are about three months old. These guys are just over four months old. Some ballpark guesstimates I have read on the forum is they should be five inches by the time they are nine months old and six inches by the time they are a year. Then you need to ask if these sizes are based on standard length (base of tail to nose) or total length (standard length plus tail). Then you have different stains which grow at different rates and some stains which "tend" to be smaller like red based strains.
My best suggestion is to watch the growout threads. The next measurements are at the six month mark and the fish will be seven months old.
Pat