I was spending time with my 2 year old daughter today and she kept asking me what, why, when, where and who for the entire time I was with her. She asked the same question a million times even though I gave her the same answer all the million times.
The key is she never was shy to ask any question and she never got bored of asking a question. Which made me start to wonder - Do kids learn so fast because they are uninhibited in asking questions - they ask them with no fear. They have parents/grandparents/guardians that would answer the questions with patience till the answer is well engraved in their minds.
Learning is the key to growth. And learning happens only when you have questions and I feel that as we grow older we have so many hang-ups in asking the questions. When we stop asking the questions we inhibit learning. On the other hand as we grow up the number of people that will willingly give you answers all the time just like a parent/grandparent/guardian gives answers expecting nothing in return is also limited.
I feel this is the reason that as we grow older the speed at which we learn things also slows down.