Average humans struggle with calculus because we have instructed average humans that calculus is hard. If we taught it to 12 year olds as a routine matter, average 12 year olds would know calculus.
You can teach smart (not average) 12 year olds the basic rules how to compute derivation or primitive function, but I doubt they are capable of distinguishing between, say, continuous and uniformly continuous function. Which is actually pretty important when trying to reason your way around calculus.
