It's amazing how many things there are that small children can learn readily, where adults have a difficult time learning. Languages, musical instruments, even things as mundane as people's names.
The loss of neural plasticity as we age is certainly regrettable at first glance. But it's not all downsides, and it can hardly be summarized with a flip, "People are dumb." There's evidence to suggest that it's actually an annealing process that's necessary part of staying smart and perhaps even achieving mastery of one's specialty. People who work in machine learning are all familiar with analogous concerns such as overfitting.