Cichlid colour depends on all kinds of factors, not just age/size.
Colour tends to be stronger in dark tanks, and will usually be washed out in tanks with strong lighting and/or coloured gravel substrates. Adding floating plants helps produce shade, and swapping coloured gravel for standard gravel helps with the latter.
Mood is important, and if a fish is stressed or bullied, it won't develop the colours it uses to attract mates (usually the strongest colours) and instead retains a "fearful" or "submissive" colouration instead. Sick fish will behave in the same way, and one sign of poor health is a lack of bright colour. Not all disease is obvious, and if you do things like offer your cichlid "feeder fish", there's a good chance you'll have introduced disease-causing organisms as well. These are often very insidious, and may take months to make their presence known. Likewise, high nitrate levels, the wrong temperature, insufficient oxygen, or the wrong water chemistry will all stress a fish, resulting in poor colouration.
Diet is a key factor, since the chemicals used to produce colours largely come from crustaceans and algae, so if you're feeding a diet lacking either of these, you may never see the best possible colours on your fish.
Finally, there's genetics. With cichlids, you get what you pay for, and the quality of stuff offered in some pet shops can be dire. Colours evolved to attract mates or scare away rivals, and if you remove these selection pressures, i.e., by breeding any old male with any old female, there's no reason to expect good colours to persist across the generations. Quite the reverse in fact, since weakly-coloured fish are as likely to produce offspring as champion quality fish.
So, in short, it's complicated, and there's a bunch of factors to consider.
Cheers, Neale
