P's can began acting out very strongly at a early age.  I have no clue about being diagnosed at age 18, but my son was diagnosed at a much earlier age by trained professionals. And I might add they were right on about him.    The signs were there from age 3+.  And as a parent we think if we love them enough we can change them.  My son had plenty of love but he still is a P.  Some P's are born that way and home life can only alter this to a small degree.  He began getting the help he needed in the first grade and it changed very little about who he is today.  He become more free to practice his P behaviour during the teen age years due to the fact it is impossible to watch a teen ager constantly......but he was a P from a very young age on.     There is a special clinic for adult P's that deal with them totally different than most other types of problems.   It has proven to be a sucess in some cases.  Only if the P wants to change can they be changed.   Most P's don't.  Again I suggest everyone read "The Mask of Sanity"  written by Doctors who have done much research and work with P's.    If you have a troubled child getting them help is a better plan than waiting until they start acting out worse.  If you are raising a P you will know what I mean when I say you know it because there are too many signs to ignore.