There’s a thing called Birth Order theory and it may or may not be bullshit and subject to cognitive bias, but I will say that anecdotally it seems mostly accurate when I think of my group of friends growing up and their siblings. There are definitely some exceptions though, like my friend who is the younger brother, but is more responsible than his big bro, though we suspect the older brother suffers from some sort of undiagnosed mental illness and is unwilling to get checked out. Short answer in my opinion is yes, the older sibling tends to mature faster. I do think that the older sibling has some power over the younger ones by virtue of being older and more mentally developed while growing up. Whether they use that to nurture and protect, or abuse the younger sibling is more a matter of personality than birth order though.

For your second question about feeling immature, I think it can happen to anyone at any time. Being put in a tough situation and feeling helpless about it can make a grown person feel like a child, despite being an otherwise mature adult. For me it happened less the more I became independent. But it still happens. For example, if car issues pop up I can get the feeling of “oh shit, I need an adult”. I think personality is a big factor here too. As an only child my perception is that I feel less willing to ask for help than others, at least until I try to find a solution myself. Some friends who were younger siblings seemed to always be getting catered to, getting rides to places, etc. That might also be influenced by other factors like family’s finances.

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.