Differs from pleco to pleco, to answer both. Some may display it sooner than others. Not all go the "24 carat gold" look, some are half half and some keep quite dull. Reasons are pretty much unknown but it was thought to be water conditions etc at one time, but I think that is bull, as have seen both in both. After discussing it with oddball he let me know this...
"Usually, radical color changes observed within a species is linked to sexual maturity, spawning, and perhaps territoriality of predominantly males of a population and is termed as polychromatism. If both genders, within a species, share the same color changes and these changes are linked with maturity and aging, then this is termed as the heritability and heterochrony of polychromatism.
Heritability is the extent to which individual phenotypes are determined by their genotypes. (traits passed down through generations)
Heterochrony relates to the timing of development (age triggers)"
this thread really made me want to buy the chubby/rubber pleco that changes color and gets all fat and stuff. they are selling the true "rubber pleco" on this site for 5.99. they say its the Parancistrus aurantiacus but is it? with shipping it comes to 35 is it the real deal or not?
I had one of those about 15 years ago. Not sure what happened to him. He never developed the color of those bad dudes. I may have to keep my eye out for one...
So on the planet catfish link somebody posted they had a blue'ish looking one. Is that how they look when they are young before the morph or can they turn blue as well? Heres the link http://www.planetcatfish.com/catelog/image.php?image_id=251