There's more to it than this...perhaps some of the 18yos have intelligent thoughts, or perhaps we just often keep our same thoughts as we age. Of course, education can be helpful and it's never too late so I wouldn't put yourself down.

Don't worry about it. You're a head of the game because you're making the effort at this time of your life. My friends who have graduated university haven't picked up a book for 10 years. I assume they've forgotten everything they have learnt. You will be the most stimulating person in the room. They will be talking about mortgage rates.

I experienced something similar, and although they had a lot to learn yet, I was happy to be around thinking, 20 somethings with fresh ideas, curiousity and energy, even as a mature student in my late thirties.
Enjoy.