No Stupid Questions

I my experience, the oldest child is forced to mature faster by being required to perform childcare duties, especially in families with many children.

Younger siblings basically "get to be a kid" longer than older ones.

This is unfair to both older and younger siblings for a myriad of reasons, but both are a failure of parenting, and society as a whole. The parents should not be forcing the dynamic, and society should not be putting parents in a position where they feel they have to.

As for "feeling immature" as an adult... That is entirely dependent on what you measure maturity by. My wife and I sing nonsense tunes at each other, sometimes just look up and go "QUACK!" for no reason. We have gone to a playground to go swinging. We have gone outside to play in the snow.

I didn't get my first job without my mother's help until 20. I didn't get my license until 26. I didn't get my first full time job until 33. I absolutely hate going to a store at all, let alone by myself. Although if I'm being honest I would rather go by myself since it's faster and less chance of my wife grabbing 10 extra things because we passed the aisle and she went something snack-y.

Remember, "Autism Spectrum Disorder" is, as the name implies, a spectrum. As such everyone is on it. Most people are gathered in a general area area (the people who would be considered "normal" by someone who has a more traditional old way of thinking) I personally believe I'm a little further toward the "autism" side, based on a bunch of comparing my personal experience with others who are diagnosed. I don't believe it's that big a deal for me, as I am fully capable of functioning on my own as an adult.

A bit of armchair psychology, I'd wager you're a bit like my wife and due to narcissistic and withholding parents you likely need more reassurance than the average person. This isn't a failure on your part at all, and it's not a "developmental disability" it's just a bit of childhood trauma.

To answer more directly:

In my opinion, yes eldest siblings often mature faster. Feeling immature is probably normal, and maturity is a pretty vague notion in general. ASD in general isn't as big a deal as many people think. If you'll forgive me for saying so, your family kind of sucks.

My incredibly naive view on this is that with the first born the parents are full of energy and try their best to discipline (assuming the parents are not already dysfunctional human beings). When the next children come along they get progressively more tired and lax, or straight up angry and impatient. The younger kids then pick up these undesirable traits from their parents.

On the other end of the spectrum we have oldest siblings with some behavioral issues and the younger ones are like "nope, not gonna become like that myself". I don't feel like this group has the same controls as the other group from a study perspective.

Parents who maintain discipline with their kids through multiple kids are pretty rare and they are the unsung heroes of a civilization.

I have no sources on this except that it is this way in my family and the families of my parents friends.