Jes, the best time to show young birds is when they come into full feather and are at the peak of condition. Typically for a pullet, it is about the time she lays her first egg(s). Roosters may take a bit longer. Maturation depends on a number of factors besides the age. Feed and care, weather/climate/length of day, amount of competition in the rearing pens, lack of stress, good health or lack thereof, the list goes on.
When all the feathers are in, when the comb and face turns a nice shade of red, . . . experience will tell you when they are ready.