I'm the youngest in the family and wow.. didn't have to grow (mature) quicker or set examples for the younger ones. But did compared sometimes, on similar parameters, because my elder siblings were brighter as they were and academics was my last strength. But I grew good at sports and developed a strong love of fine arts..a different league!
The eldest child has the benefit of getting those few extra years with their parents than their siblings, but they also usually get given the responsibility to look after the younger children, and set an example. They might have to grow up quicker.
I am the oldest but my brother is way smarter than I am. I was much bossier though and very protective of him even though he didn't need it. My sister is 11 years younger so she was more like an only child in a lot of ways.
I'm the eldest of three. My sister (middle child) is the smartest though, with a scholarship in tow. I was the only child until I was 10 though, so I think I have developed a mix of both single child and eldest child traits, which oftentimes contradict each other.
I am the eldest of three sisters. Yes, there is a certain expectation (higher than others) from the eldest. However, i don't think the level of intelligence or IQ has anything to do with the order in which you were born. In fact, it is directly proportional to how hard you work to attain higher grades, etc.
Being the eldest of a family is also a big responsibility. The parents are always seeking your advice for any action to be taken . But as long as you are together with the family your brothers and sisters bear a lost of respect for you. After the departure/deaths of parents all the respect is vanished and only at the notary you know what is really a brother and a sister when it came for heritage.