children are quick to imitate and copy. But they do not have understanding and logical power. Adults are slow to imitate and learn by heart. But they have better grasping power. There is another aspect. Children learn afresh. But the adults need to brush aside the previous knowledge and replace with the new. Children's mind is like a clean slate on which you may write anything. But adult mind is like a dirty slate that needs to be cleaned before writing something new.
I do agree with the concept that children learn quickly than the adult as they are new to the learning world with lot of interst and enthusiasm to know the things in a better manner or way.
The reason children learn so quickly is mainly because their minds are completely open and are not clogged up by any kind of pre-made up notions or any such thing!
Its true about kids but they do it fast because there mind are raw and are not bundled with pre filled baggage of knowledge nor they are busy doing other important work indulging them mentally.
Its easy to write on a clean black board but its difficult to do it on already filled one.