Well, guppies can take anywhere from 2-4 months to sexually mature. There are different factors involved that will impact the rate which they mature. Temperature, food, tank mates, etc. The temperature combined with food will have the greatest impact of all. The higher the temperature is at the faster the guppies metabolism is running. So if you give adequate food for the rate that the guppy can eat it, the faster the guppy will grow.
The tank mates also play a huge roll, if the young guppy is scared to get the food it will eat less, no matter what amount of food it’s offered. The temperature will have no impact on the young guppy other then it will feel very hungry. Now you may be saying well I bought a guppy that was younger then 2 months of age and without a male it gave birth. The guppy has a really neat way of making sure that no matter what its kind will continue to swim for eons to come. What the female does is she will store sperm for up to seven months inside of her. What she does it that when she feels it’s a good time to give birth she will use the stored sperm to fertilize her eggs. This way she will have readily available amounts of sperm for almost half her life span. To me that’s pretty incredible.
So here is the run down on when they mature.
From birth to 1 month they are fry.
From 1 to 2 months they are juveniles, so teenagers,
And if all the conditions are right, the food is sufficient, and adequately nutrient, the guppy will be fully mature by the middle of the 3rd month.
Now there is a difference in the actual time from when a male is mature, and when he is able to reproduce.
So what I mean is a male can start to reproduce at the young age of 1 month. Now what that means is that if you have mature females in the tank with a 1 month old male that he is going to impregnate the female and you can quickly become overwhelmed with guppy fry. But if you are housing young males and young females together then you will be fine, a female can hold sperm from a male for up to 7 months, and she can hold it before she is mature enough to fertilize her eggs. And since females aren’t sexually mature as soon as the males are, they sexually mature around the 3 month period, you will not be over run by guppy fry.
Now fully matured guppies are just full grown and have all their color. That’s what takes place around 3 months.
Sexual maturity is different:
Males 1 month, and females 3 months.