Youngest children are the most selfish of all birth orders. They have been catered to all their lives by their parents. If not their parents, then their older siblings cater to them. In multichild families, particularly in larger families, youngest children are pampered, indulged, & assume little or no responsibility.
Youngest children also are treated the most preferentially in all birth orders. They are allowed to get away with things that their older, particularly oldest, siblings would be chastised, even punished for. It isn't unusual for youngest children to do things yet the oldest/older siblings are blamed for what youngest children do. They are adept at the art of manipulation, playing the innocent part. Of course, they are believed because they are the baby of the family.
Because youngest children are catered to, they don't have to learn the art of giving. They expect the world to come to them. They also don't know how to be empathetic to others because they were always on the receiving end in the sibling relationship. They never had to adopt the proactive or sacrificing role as their older siblings have done. They will always be the center of the family universe & will never be dethroned as their older siblings were. Yes, many youngest children are what one would deem insufferable, even bratty. It is THEIR way or NO WAY. They aren't good at reciprocating when it comes to relationships. In sibling relationships, youngest children are, on average, takers but seldom or never givers. It is THEIR universe so to speak. They are the GIMME, ME, ME, ME birth order par excellence.
In many adult sibling relationships, it is the youngest sibling who never have to make sacrifices & have the easiest life. For example in larger families, youngest siblings are the ones who can attend college as they have older siblings oftentimes foregoing education in order to work to supplement family income. Youngest siblings have it the easiest in multichild families, particularly larger families as opposed to older siblings who had it much harder.