Can An Adult Pass A Middle School Science Test?

Sometimes.  Maybe even more often than not.  However, after a few years, if you don't use what you learned, you can easily forget most of it.  I couldn't tell phloem from xylem anymore, since I haven't once needed to recall that information in the last 20 years.  (Well, not that I remember anyway. :-P)
 
Also, the way that things are presented may have changed so much that we wouldn't understand what the teacher is getting at with a question.  My husband and I took a while to figure out my stepdaughter's middle school math a few weeks ago, not because we can't do math, but because we found the way the problems were presented really bizarre.  Once we figured out what the teacher wanted, it was simple, but we needed the text book to understand what she was asking.
 
Also, some things have changed since some of us were in middle school.  Eg. Human  understanding of the atom was improved a lot between my parents' middle school days and mine.  And what my kid will be learning in a few years about the atom is different from what I learned in middle school.   I might never have caught up if I hadn't decided to go back to university and study in a field that requires chemistry.

I'm doing my science fair project on this and so far I learned these things.

Adults might be able to. It really depends on their job. If your test was just on math, a math teacher, or someone who does lots of math at their job will probably pass with flying colours.

There's a better chance for a teacher to pass then any other adult considering they're kinda suppose to know this stuff.

And most of the tests I got back told me that most adults passed but a lot of them barely passed.

So in the end, it really depends on the job the person has.

Well I am doing a science project on it maybe, and I can't find any info on this >:(

No or probably not it has been years since they've been in school. The probably forgot and they have enough to worry about with jobs and stuff.

Depends. If they are using it, like my sister in university, they would pass it with flying colors. But if you are like my dad, a salesmen, you probably would not as well. My mo didn't under stand my science at first so I think if you are not using it, your brain does not find the need to keep knowledge.

Absolutely not because I just did a science experiment on it and my hypothesis was that they could but I was wrong