Not a good plan to use the Boston bombers as an example of why vetting wont work.  The Feds were getting all sorts of red flags on them and ignored them.

Why do people insist that teens and children are harmless?  They aren't.  There have been child soldiers for centuries; bullets fired by them are as deadly as those fired by adults.  Teens are probably easier to vet since they stupidly put their entire lives on the internet.