My 16 month old boxer bitch has recently turned aggressive towards my 14 year old yorkie dog. It seems to occur when visitors knock at the door. Up untill now there has been no problem between the two of them but is getting to the stage where she is attacking him.
Any help is welcomed.

If it only occurs when visitors knock the door then I would separate them at these times.  It is probably just excitement.  If he barks, perhaps that winds her up.  Better to avoid the problem than to try to find a cure and fail IMO.
Sally

Hi Guest

As Sally says it is probably excitement.

Change your "answer the door routine".  First get a bell and put a cloth around the knocker so you change the noise.  Next when someone comes to the door take the time to separate the dogs (put a note on your door that it can take a while to open it) and do all this very calmly and quietly with no excitement or sudden moves on your part which will wind your boxer up even more.

Is she spayed?  Could she be approaching a season?