aggression in GSD

Question
I adopted a GSD - Max is the most wonderful dog around.  Gets along well with the 3 cats.  I've had him now for about 1 1/2 years.  He is @ 8yrs old.  we found recently that he is getting more a more aggressive to people when they visit.  It use to be he would stop after a few minutes now he continues and much more aggression if showing - he growls at them.  He use to just bark and then stop when asked.  Is this an age thing?  How can I curb the behaviour?

Answer
I don't think it is age.  He may be filling what he sees as a leadership vacuum.  The key to most behavior problems is approaching things using the dog's natural instincts. Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog. Life is much easier if the 2 legged pack members outrank the 4 legged ones. You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/ For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm

At his age, training may go slower.  You may need a private trainer.