For reasonable definitions of "less smart", yes, you were. There's a lot of work on child development, and it does not support the idea that an 11-year-old is as fully equipped as an adult. Piaget's child development stages is an accessible overview if you want a nice Googleable term. Modern academics would quibble with various aspects but from what I can see a lot of the general ideas one would get from a quick breeze-through are still a reasonable start.

I'm actually pretty close to first in line to say that we underestimate children... buuuut, at the same time, no, they are not just little adults that are getting suppressed by The Man or whatever. They really aren't anywhere near fully developed yet, and are as a population, generally incapable of many things no matter how hard you tried to push them. And even if you do find an individual 11-year-old that, say, is fine with calculus, in another 10 years they'll be even more developed.