I agree,and it's an annoying cliche that was so perfectly sent up in Scream,13 years back.But it's been this way for years and obviously falls in line with what the marketing men suggest,because the majority of the cinema audience,are either young or male.
It maybe goes back to the Texas Chainsaw Massacre and Black Christmas,but If truth be told,it goes back to the B movies of the 1950's.Halloween and Friday the 13th kick started this into the modern era,due to being very popular among teenagers in the late 70's and early 80's.
Either way,it's all fairly irrelevant because out of 100's horror movies,you'll find a gem like The Exorcist or Psycho.Hollywood being run more and more by marketing men,coupled with horror being so cliched,and directors being told more than ever what to do,makes the chance of finding a good horror even more difficult.Especially one that doesn't feature a dozen 25 year olRAB playing 16/17 year olRAB
I do find it ironic how the Saw franchise,despite featuring few actors under 30,has become probably the biggest grossing horror series in history,having a huge teen following.It goes against what Hollywood has been doing for decades.