They can start breeding from 3-4 months old and the females have less colour on the body, but the may way to tell is the males have a gonopodium, this is the organ used in mating and these develope between 3-6 months old in males.
how big are they at 3 months? right now i have a male and three females ( the male im guessing because he is the only one with colour) but they are still pretty small
Fish growth varys depending on feeding and environment. I've had fry become full grown in 3 months time, and iv had fry become full grown in 6 month's time. Parameters, feeding, space, genetics; it all plays a role. And with domesticated fish such as the guppy, there is no precise answer. I'd say my females start becomeing pregant at a little over half an inch long, and my males are ready to go as soon as they develope the parts they need to do so; at about 1/4 and inch.