An adult is someone who has lived for 20yrs or so, experiencing and learning many things in that time. A body just zapped into existence, even if made to look like Methuselah has not had the experience and knowledge that make someone an adult.
Interesting how you mention 20+ years or so, because in Scripture often a person of experience was about 30. such as: when Jesus started his ministry.
There is nothing to Indicate that Eve was created at the same time as Adam.
Thinking of names for the animals could involve also thinking of their characteristics before applying a name to them.
Unlike a small child making up a name, in the Hebrew language often names had meanings attached.
Possible that Adam could have easily been 30 years old before Eve was created.
Also, we do not know how many years passed after Adam and Eve were married and before Eve sinned.
When Eve was created and sinned is a key that is missing in not knowing the day or hour of Matthew 24v36.