Quite a lot of condescension here towards people trying to make a better life for themselves. Were you all really that smart at 18?
A lot of it, I think, is cultural. Growing up poor but with a shot at college, you tend to make choices to not be poor. If you grow up affluent, there's a lot of "follow your dream"/"do what you love" despite the probability of maintaining affluence. So you end up studying musical theatre or Celtic Studies and working as an articulate barista.
So yeah, some of us were that smart at 18.
If I wanted to learn something, I was pretty much on my own. I found school to be largely a waste of time but I had little say in attending elementary, middle and high school. I still felt the same way by the time I was graduating college, but I wasn't quite ready to enter the Real World and college was a good excuse to avoid it.
I received a Pell Grant for the first semester of college, but the paper work was insane so I ended up working my way through college. In this, I was fortunate that cheap (as in "too easy to get") student loans had yet to flood the market with money so tuition wasn't insane. That, and I attended a Florida State college as a resident (cheaper than going out of state) and it was only a few miles from home so I saved money there.
Part of the reason for gullibility at that age is no incentive to work really hard in school and trying to understand how adult life works. Grades instead of ranks, no fear of falling into poverty or survival mode (as would happen in a poorer country)
There is no reward for 'think and proceed'. There is reward for 'go with the flow' and that is temporary respite, not having to struggle, having fun etc.
At a systemic level, kids have to be encouraged to compete and become better AND demand better from others to be able to succeed.
I used to think I'd become a writer and write a book like Harry potter. At age 10, this naivety is cute. At 18, that literally is money (and possibly future life).
Schooling really needs a revamp.