I am the baby of my family. My sister is the oldest and brother is a middle child. It's hard to pinpoint what characteristics come with this, but you know them when you see them. I'm sure it has everything to do with how much parenting experience your folks had when you were born.
The oldest child: If the circumstances were good, the first born had two freak show parents. Neither parent knew what to do or what to expect. A cough signaled certain Pneumonia. A little crying was extreme colic that warranted a trip to the doctors office. The first baby eats every 2 hours on the dot and every whimper is answered with mommy's attention. Far too much attention was paid to the oldest in the early days of their lives and mom and dad had to learn how to parent using the eldest for all their trials and errors. There would be MANY trials and errors. Long before the parents have a clue about what they are doing, the second will be on the way and your solitary life of attention will come to a halt. The upside is that all your clothes were new! You're younger siblings will have to wear all the crap that you happily stained.
The middle kid: This kid is temporarily the baby so all is well. This kid has to be amazing, super cute, or of a different gender than the others to keep mom and dads attention. This kid needs to resort to extreme good behavior or super naughty behavior to keep the spot light. If mom and dad get busy with the next baby, it will be fine, because this kid has an older sibling who is more than happy to boss this kid around. The middle kid will never have the insane amount of solitary attention that first born had, and sadly your spot as the baby will be ever so temporary. This kid is lucky because mom and dad aren't complete freak show idiots anymore, and they are busier now so you can get away with a bit more than the oldest could. If the middle kid gets frustrated with their plot in life, they can take it out on the baby (as long as mom and dad aren't watching).
The baby: Oh the baby! You will always be the adorable little, trouble making baby. Your siblings will love bossing you, yet they sort of resent you for stealing some of their attention. The rules will be very relaxed for you little one. The curfew for the oldest was 11:00 and you can feel free to stroll in at 12:45 because no one is awake to notice. You can fly under the radar. Mom and dad aren't overly worried about you because they already know that everything will work out in the long run. Who cares what you eat? Go to bed whenever you like. You are the baby, and you can expect to get your way. No one has the time or care to stop you.
I don't know exactly why, but I look back and notice that I almost NEVER dated anyone who wasn't the baby of the family. I married a youngest kid and strangely every boyfriend I ever had was the youngest of their siblings too. I don't think I ever once even had a crush on an oldest child. I wonder why this is? Now truthfully, I can't remember each and every one, and I'm not sure if the regular rules apply in blended families, but really I've only ever liked the last born kid. Why? I'm not exactly sure.