I don't think that's too fair a comment. I happen to be one of those youths (albeit, with a bit more taste), and I know plenty. I've definitely declared all of the little things that I've changed on my car, even my stereo. My friend has a roll cage in his car, and there's only about 8 insurance companies in the UK that will thus insure him... he still went through the trouble of finding them and declaring it.
I will agree, however, that some of the cheaper sheds are not declared.
I just would rather not be told that I haven't declared my mods
Dom