Well mostly the little guy (16 months)...he has major issues with not getting his way. This weekend my husband was reading an Elmo book to the little guy and the 5 year old came over to listen. He didn't even go near the book he was just standing off to the side listening. The little guy turned around and grabbed the 5 year old and pushed him to the ground. Yes, my 16 month old is beating up his big brother over an Elmo book. The little guy got in trouble, but really how much impact does it make at that age anyway???
Later I was reading to the little guy while his brother was watching a movie. Every time the 5 year old would look over at us the little guy would slam the book shut and then 30 seconds later he would be saying "book, book" for me to read to him again. This of course became a game for the 5 year old.
Honestly I thought it would take a little more than 16 months for the boys to start tormenting each other, especially since they are 4 1/2 years different in age..I guess that was just wishful thinking.