Every time I tell someone I'm the oldest kid in my family, I get the typical eyebrow raise as in "wow that's cool" (even though they probably don't care) and then the follow up of questions about how old my other siblings are, everything they want to know about my family, etc. as if I'm irrelevant. Now, I'm not sure why this happens, but I like to think it's because whoever asks me this assumes I was the most responsible and grown up. So let's just go with that...
Even if you're not the most responsible or grown up person in the family, being the oldest child sort of sets you up for these kinds of expectations; take it or leave it, even though you can't really leave it. Whenever Mom and Dad were gone in any situation you had to take things into your own hands whether you liked it or not. Say your mom runs into the grocery store and you and your little brother are waiting in the car when all of the sudden he starts making this obnoxious pig noise. You just look at him with those eyes of hatred because you know he did it just to bug you. Being the oldest kid, you have to be mature and just let it go and ignore him. "Be the bigger person," my parents always said.
On a more important note, when more meaningful things fell into your hands, you always knew it was your responsibility to do the right thing and set an example because that's what was "expected" of you. Like showing your younger siblings around your high school, even if you thought it was embarrassing, or bringing them with you when you hung out with your friends because your mom said so. You had to be the role model.
Another thing that made me cringe when I was a young teen was, "Oh, can you just watch your brother tonight for a couple hours?" "Hmmmm... mhhhmm yep. Sure." You were lucky if you were clever enough to request a reward for these hours of crazy babysitting. Your younger siblings knew and still know how to push your buttons the most, so never forget that. But down the road after many, many, many fights, you became a professional at being responsible for someone else other than yourself. Even further down that road you may have, just may have actually made a place in your heart for them that isn't all evil paybacks and torturing.
After many years of being the oldest sibling, not only have you built up patience and responsibility, but your younger sibling(s) will someday realize all you've done for them and you can hopefully have a close relationship with them; what comes around goes around. You're probably more mature than other kids your age if you grew up babysitting and being the second mom or dad of the house. That's something nobody can take away from you, so be proud, oldest siblings!