As a personal opinion, I do not feel that it is fair to ask anyone to put their age on display in public like that. Anyone who comes online seeking advice should be aware that the person they're getting it from might not be an expert (even if they sound knowledgeable and articulate). Additionally, it's not logical to assume that someone is less knowledgeable about a topic simply because they're young. Obtaining knowledge is about study, something a person of any age can do. That's why you get adults who struggle to help their grade school kids on math homework. How much knowledge a person has is about their proximity to it--how much they pursue it--not about their age. You might be inviting people to ignore good advice from the Fish Guy simply because they see his age.
Finally, and most importantly, it can be dangerous for a kid to publicly advertise their age online. This is a friendly forum full of great people, but there is no guarantee that less savory types might make their way onto this forum. I personally feel like is unnecessarily risky to ask the Fish Guy to display his age, and I do not think the concern that other people might unwittingly take advice from a kid is a big enough concern to justify potentially putting him at risk. Especially as he has already proven that he has an honest, trusting nature.
The laws about children not being on forums is there to protect the children, not to protect people from following potentially ignorant advice--or to protect experts from being annoyed at non-experts for providing bad advice.
I say let him stay on the good faith that he's been honest and that his parents know he's online, but don't ask him to share his age with everyone. It just seems wrong to me.