The thing with celebrity is that you typically have to have done something with your life to achieve it (that involves time). When you turn up on celebrity big brother years later it generally means you've failed at being a celebrity/need some extra cash/ or you're retiring from it; so by definition CBB will always feature 'older' participants.
Don't be too surprised, 16-25 year olds generally aren't important or interesting as evidenced in other big brothers.
I suppose the exception to the rule was danielle lloyd sp? last time who proved that dull 18 year olds really don't have enough life experience to know right from wrong.