There is a popular myth floating around, which suggests that children are somehow more capable of learning than adults are. People love this myth, because as adults it gives them a convenient excuse for not learning.
The myth is particularly popular in the realm of language learning. It's just accepted as true, without any argument, that children have some sort of amazing, otherworldly ability to learn languages, and that somehow that ability is lost as we get older. Really! People believe that some biological function for which we are evolved can, somehow, just magically disappear when we reach the age 16!
Sorry, I don't believe in magic.
These are the real differences
The differences between how children learn and how adults learn are stark and unmistakable. Children are surrounded by input when they learn. There are speaking aids: repetition, gestures, pictures, etc. Many people even use a "dumbed down" version of the language when talking to children. And yet we don't do any of these things with adults.
Children acquire a language. It's presented to them through use. It's situational and relevant. Children hear the word "eat" and are presented with food. Children hear the word "apple" while being offered a delicious red fruit. They hear the word "no" in an angry voice as mom swats them on the ass. And yet as adults, we get a list of words and a book of grammar.
Children are always immersed as they are acquiring their first language. It's not compartmentalized into a time of day, and a particular book, and a lesson and a strategy. They learn because they can't escape it. They learn because it is necessary to survive.
Children also have no past experience to compare it to. Everything they learn is being learned with a completely open mind, whereas adults are constantly trying to apply their current assumptions and expectations to any and all new information they receive.
The myth is... teaching!
The reason adults don't learn like children is because they believe in the myth of teaching. In other words, it's ego. It's the foolish belief in convenience and shortcuts. Adults don't learn because they think they can take a shortcut and "be taught". You can't.
“If I had read as many books as other men, I should have been as ignorant as they are.” — Thomas Hobbs
I've said it before: language is not a fact. It can not be taught. It can not be learned through study. Language is a skill. Skills must be acquired. Skills must be practiced. Children learn to talk the same way they learn to walk. You can't be taught walking. You have to figure it out for yourself. The same thing is true with language.
Sure, someone can help you keep your balance, and they can even force you to put one foot in front of the other, but until you do the work of operating your own legs, and until you build the strength, the balance, and the stamina you will not walk. Not on your own. And likewise, until you build the vocabulary, the grammar, the hearing, pronunciation, and comprehension, you can not talk.
Books are handy. Grammar is a convenient road map. Classes are comfortable. But these are just the language-learning equivalents of mom or dad holding you up by your arms while you kick your legs and learn to put one foot in front of the other.
Eventually, you have to stand on your own. And indeed, eventually you have to fall on your own. You are no different than a child in this respect. The difference is a myth.