I equate smart with maturity and experience. The more mature you are, the fewer stupid mistakes you make. When I was 12 I thought 8 was immature and more prone to do stupid things and I was, of course, right. The only thing is I also thought my parents were stupid in which case I was, of course, wrong. The older you get, the more experienced you become and the less stupid you are. Yeah, I know there are exceptions but those people are just extraordinarily stupid. To quote Mark Twain:
"When I was 18, I left home knowing my Father was a stupid man who knew nothing of the world; then when I returned at age 21 and sat down and talked with my Father, I was amazed at how much he had learned in three years..." Then there's experience. Some people have to repeat grade 5 two or even three times before they have the requisite experience to advance. You could uncharitably call such children stupid but unless they've got a legitimate learning disorder or domestic problems your comment might not be too far off the mark. I think this circumstance is genuinely rare. I have met very, very few truly stupid people.
So what's left are the cards you're dealt. You, my friend, have been dealt some really shitty hands. Nothing to do with your gray matter. Everything to do with probability theory.