Why does Young Sheldon behave like an adult while Adult Sheldon behaves like a kid?

I'm not an expert in the two series, but Sheldon Cooper is supposed to be an eccentric genius. I believe that the writers say that they don't use Asperger's syndrome as a basis for writing Sheldon, but many viewers think that Sheldon has it. So Sheldon is very unusual as well as being very intelligent.

A precocious genius child is likely to seem very adult compared to most kids his age.

Adult Sheldon often behaves erratically and childishly. But he is a theoretical physicist at Caltech and thus a successful member of society. He is as adult as society needs him to be. So he would probably be annoyed by people who want him to behave more like a typical adult just because it is more normal. Normal behavior for normal's sake, instead of normal behavior in order to have better results, would seem senseless to Sheldon. Requiring Sheldon to be normal just because it was normal would seem like an act of tyranny to him.

So I guess that Sheldon has changed a little, but not very much, between Young Sheldon and The Big Bang Theory. I suspect that his similar personalities in the two shows seem different because 9-year-old Sheldon often seems extremely grown up compared to kids his age, while adult Sheldon, having successfully adapted to adult life, still retains a lot of his childhood personality traits and thus seems immature by comparison to most "normal" adults.

NOTE: The question itself is opinionated, so would my answer be :)

Rather than comparing Sheldon's behavior to a normal child behavior and a normal adult behavior (in both his childhood and adulthood), we should compare it to his “practical” approach towards any situation.

Moreover comparison should take place between the young Sheldon and adult Sheldon, when he appeared first in The Big Bang Theory. I mean the motive of the two series is to fill the gap of Sheldon’s life between the two.

It’s obvious that one’s behavior would change overtime (no matter how Sheldon they are), and Sheldon should be no exception, as it is evident from the whole The Big Bang Theory drama (12 seasons).

Even with that in mind, if we compare Sheldon from Young Sheldon and The Big Bang Theory, he has changed very very little. He is still afraid of animals, birds etc. He is still a loner. He still finds a way to connect everything to Science. He still would not hold hands or bear hugs. He still keeps his Potty training journal. And the list goes on.

Creators can not be given more credits to what and how they have achieved the similarity between the two Sheldons. Other factors that might have changed in his behavior can be assumed to be the unavoidable changes growing up mentioned above, which are yet to be shown in the gap between the two series. For example how he changed from wearing formal shirts to comic t-shirts is something I would like to see too. But dressing style should not be counted as behavior, or should it...?

I think that young Sheldon had matured according to the children in his age group but when he grew up, he found Leonard who was kind of childish and also mature as seen in the show, so Sheldon didn't feel a need to act very mature and that is why you can see him acting that way in front of his friends, Rajesh, Wolowitz and Penny. Later when he meets Amy you can see the way he acts is very adult like but later when they get close he opens up and acts like his true self.

It's a simple matter of different writers. If the the premise of Young Sheldon is to be more of a Sitcom like older Sheldon, he would be off the walls bat crazy like his adult self. To me, Sheldon is very much the same in some aspects even though his overall personality is totally different.

I've only seen parts of these shows. The Sheldon dude has a high functioning form of Aspergers(it covers a wide spectrum). He don't know or always know how to socialize in a way that's going to be pleasing to other people. Not meaning to be rude, but it could be viewed as that way as his speech doesn't factor others into it usually(usually deductive reasoning translated to speech without the filter to say "don't say that or cut that part out). Only speaking from experience here...

For his behavior from child and adult, they actually looked the same to me. The parts I saw of young Sheldon figuring or doing things using his intellect, also were the same things the adult guy did. He tried to speak more adult as a child, and as an adult maybe seemed childish because of the things he was into. But that's just a person liking nerdy things. He was more serious as a kid because he hadn't learned to have fun too.