Part of the nature of language is that young people almost invariably want to have their own 'language' - or more correctly, their own meanings to words. This distinguishes them from the 'older' generation, so they have their own 'club' as it were.
Back in the sixties (
1960's in case anyone is
wondering being sarky

), words like 'groovy, cool, hip' etc came to be used by the teenage group (errr .. so people have told me

), thus giving them a common sense of identity. It is the same these days - only it's 5-10 year olds as well

. Hence 'sick' meaning really great etc.
Our irritation with words like sick etc just shows that we are getting older .... we are no longer the 'younger generation'
I haven't been part of the younger generation for quite a few years now ... 