Oldest children are considered to be the strongest of all birth orders bar none. They oftentimes have to endure & experience things that no other birth order has to experienced. They have it much harder than other birth orders. They have very high, even stringent expectations placed upon them. They aren't allowed to be normal children as they must set a perfect example for their younger siblings.
They must be adult-like from an early age as much responsibility is placed upon their shoulders. They have to toughen up, be strong, & aren't allowed to be vulnerable as it is a seen of weakness & they mustn't be weak in front of their younger siblings. They must be there 24/7/365 for their parents & younger siblings. They are THE SHOULDERS that the family stands on. They are to go-to person whenever there is a problem in the family. They are ones who everyone in the family, including the parents, depend upon.
They also must sublimate their needs, wishes, & desires to that of their family. They are told that their individual needs, desires, & goals are secondary to that of their siblings. They even sacrifice part of themselves for the so-called betterment of the family. They are seldom or never allowed to pursue their individual path. In wealthier families, they must follow the prescribed educational & career patterns set by their families whether it is business, medicine, lawyer, or other professions. In poorer families, they often have to forego their own education to supplement family income & to pay for their younger siblings' education. It isn't unusual for oldest children to continue to financially support their families as adults, oftentimes shortchanging their own spouses & children.
Because of strict standards & having responsibility at a young age, oldest children are very authoritative, even authoritarian in their dealings with others. They aren't of the democratic vein. They are controlling & rule/supervise with an iron hand. They aren't one to suffer fools gladly. They aren't loathe to use methodology of power=might=right. They love to use power plays to get others in line. They are also very exact in their punishment if they have been crossed.
Another aspect in their having control is being strong & invincible. They feel that it is quite fallacious to show weakness in any form. To show weakness means being disrespected & they don't want to be disrespected. They are to be respected at all times, if not adulated.