The Canadian Medical Association Journal has published an article on why Tintin never aged during his career. The study revealed that this was because he has repeatedly received blows on his head which triggered a growth hormone deficiency.
What a waste of time, but its still interesting. Apparently this is not the first of CMAJ's stories into ailments of fictional characters.
In 2000 they revealed that Winnie the Pooh's obsession with honey was because he suffers from psychological issues among them Attention Deficit Hyperactivity Disorder and obsessive-copmpulsive behaviour.
Winnie's friends did not escape the doctor's assessments either. Piglet was deemed to suffer from anxiety disorder and needs anti-panic medication while Eeyore was chronically depressed. Tigger was deemed to be hyperactive while Christopher Robin has gender issues and needs parental guidance.
They also concluded that Squirrel Nutkin, a creation of Beatrix Potter, was autistic!
Source: Yahoo! News