You only notice the middle aged ones because they can afford the posh bikes. You don't notice the yoofs with supersports because they can't afford to buy or insure them. Economics has largely turned biking into a leisure activity rather than a basic need for transport issue. And an expensive leisure activity at that. And the folks with the largest disposable income coupled with a vague recollection/yearning for youthful persuits are the middle age folks.
**** - just thought, is it still "three score years and ten?". In that case I've only got two years to go before I'm officially "middle aged"
Ouch.
Besides, as has been pointed out, many of the middle aged bikers around have earnt their biking stripes on decades of two wheeled ****e, now they can afford something decent can't we at least let them be and forgive their terrible fashion sense?