Great answers thus far. What comes to mind is nonglamorous, drudgery, unappreciated, overused, Mr./Ms. put upon, pressed into enforced, neverending, unending service to parents & younger siblings, no privacy, no individual life, no childhood/no adolescence, putting other's needs constantly before one's own, seeing childhood/adolescence as a prison hoping to escape from ONE DAY & by any means necessary.
Having to forgo higher forms of education to work & help support family of origin. Being the REAL parent in the family as the adult parents are parents in name only. Being constantly cast aside if not pressed into service. Never receiving parental affection because it is deemed unnecessary for the oldest child. Being needy because of never receiving adequate/individualized parental love & attention. Being depressed because of inadequate parental reinforcement; even being suicidal (know of oldest in large families who ARE suicidal, wishing they were dead because of a HELLISH life).
Living an UNIMAGINABLE HELL on earth which no child should ever endure. No such thing as childhood. What? Childhood NOT for you, you have diapers to change, house to clean, food to cook,etc. Looking at other children enjoying a childhood/adolescence, wishing THAT was YOU. In essence, living a Cinderella existence w/o the stepparent. Oh no, your parents are THE STEPPARENTS in this scenario. ESCAPING from home at the FIRST opportunity, whether positive or negative, mostly less than positive.
Forestalling, even abandoning your goals because parents & siblings NEED you. Supporting parents & siblings financially, not even getting the needs you want, putting yourself LAST. Putting your family of origin before your own family, much to the consternation of your spouse & children who wonder that your siblings should take care of themselves & not ALWAYS depend upon you.
Being the oldest in a large family is something no one would wish on their worst enemy. It is something that NO ONE in his/her right thinking mind would want to be....EVER. The lowest circle of hell is FAR MORE PREFERABLE than being the oldest child in a large family!