SSBI background check?

I am going to have to get a top secret clearance for the job I am going to be trying to get in the air force, kind of curious how it would work though since I am 18? I have been told by many many people that they will only go back to my 18th birthday(in my case). But I guess I just don't think that sounds like they would be satisfied with such a small amount of info (i.e. one high school, 2 addresses, no jobs). Was wondering if someone could shine some light on the situation, like if its true, or if they would feel the need to go back to a younger age? I have nothing to hide, just very curious.