Size is more of the determining factor rather than age. I’d say around 1.5-2in or so depending on the species of mbuna. Many mbuna already have adult coloration as juveniles, such as yellow labs and zebras. At 2.5-3in you should be able to easily determine gender based on coloration and behavior.
I think my peacocks took a good 8+ months before they could spawn and that was right around 2-2.5in