I'mma try to decipher this....you're asking when do the bellies turn red? And when do they turn darker? They get red as they transition from babies to juveniles, they turn darker as they grow older.
You could also be refering to a rhom which never had red
aside from age, it also depends on your water condition and diet given to the fish. they can retain the red belly even at pretty large size if cared properly.
These pictures were taken at Mystic Aquarium, CT and I'd say they are easily 10"
Great lookin' photos - the stripped fish look like Leporinus Fasciatus " Banded Headstander " originally I was thinkin' Distichodus of some sort but they are from Africa not S/A - wouldn't make alotta sense - Just a guess.