I didn't even know that
P. charus existed until I searched fishbase, so I really do not know.
That your fish is 2 years old and only 8 centimeters would to me suggest that either the final size is smaller than the cited 20 cm, or that it's a slow-growing species. But that's just conjecture on my part.
On a related note, I see that
P. charus does not have a cat-eLog entry or any photo in Fishbase. If you could take some more photos I'm sure both Fishbase and Planetcatfish would be happy to add them to their databases.
For Fishbase I'd suggest mailing
[email protected]; I do not offhand know where to send photos for planetcatfish - anyone know?