junior member ?
 

at the very real risk of appearing thick,why do I show up as a junior member?

Because we all were once:) After so many posts you become a member and later I think after 50 or 100 posts you become a senior member. I think when you are no longer a junionr member you can change your title to anything you like, in User CP > Edit Profile.
You dont have to be young to be a junior in these forums:)