As the oldest child, you experience many different roles, experiences, and test-trials than your other siblings. Parents are figuring out the child-raising process and as the oldest, you run into things that your siblings will never experience. Here is what I have noticed throughout my years as the oldest.
You are the entertainment. My parents treated my like a small plaything when I was younger. My parents will admit to putting me in an inter-tube in the bathtub and then turning the jets on solely because it freaked me out. I was like 1, maybe 2 at the time. It is just the two of them plus you, so as the only child, you are the sole form of entertainment as their day winds down, whether that be feeding you weird things or finding ways to get a reaction. By the time the next child comes around, it isn't nearly as fun for them and they have to divide their attention. You also get exposure to a full range of activities. I was enrolled in all sorts of classes like Gymboree and a bunch of different sports to keep me and them busy. My parents figured out what they liked or didn't like, but I got to relish in every minute of Gymboree and gymnastics because it was only me. By the time my sister rolled around, there was more picking and choosing to prevent both of our activities from overlapping.
You are the constant babysitter and second parent. How many of you have been in some sort of circumstance where your sibling ends up hurt or making a mess and somehow it ends up being your fault? Yup. But because you were in the vicinity, apparently you should have been watching. They could literally be crying over absolutely nothing or have smothered themselves in mud, yet you get blamed for doing your homework and not eyeballing their every move.
You are the test dummy. You are the first child and your parents essentially have very little idea what they are doing. You are the trial and error for literally everything involving a child. Daycare? After school activities? Food? Money? Drivers ed? Age for ear piercings, cell phones, and sleep overs (all of which got less for my sister...)? Sports? Sleep schedule? College application process? First job? I am sure the phrases, " I don't know, we will see how it works" or "Well, now we know how this works," have been used on many occasions. My consistent phrase in regards to this factor tends to be "Wait I didn't get to do that until I was _____ age," while my siblings got to do all the fun stuff years before. By the time the second child rolls around, parents normally have a little bit more of a system.
You turn into the third "working" member of the family. You are the go to when something needs done. At first, it is that your younger siblings aren't capable of certain tasks, but as you all get older, it turns into your siblings don't know how and you do. This would only be because you have been doing said tasks for years. I then attempted to teach my younger siblings different chores and let them take over after my years of hard work (kind of...). And then the real kicker is that your parents ask you to take back over because the younger siblings just can't do it as well as you can. Typical.
At this point in time I am still paving the way as the test dummy as my parents figure out how to handle me becoming a self-sustaining adult. As they figure out at what age to cut me off (hopefully around 27, lol) and how to cope with me just growing up in general, my siblings will know what to expect during this weird growing-up process.