Espanol y El Cerebro
As a three year old child your brain is still developing. It's like a sponge. When a baby is exposed to a langue all the information that they are taking in is all new information. The advantages of being a young learner of a new langue is, you are more likely to develop a native like accent.
As a 18 year old it is a little bite more harder to learn an other langue, why? because your brain has already developed and adsorbed your first langue. Your body has already gotten use to speaking, thinking and learning your first langue. The advantages of being older and learning a new langue is you get to control how much you learn and what you learn and how you learn it. It takes 2-3 years to be 100% fluent. It takes 4-10 years to be 100% with speaking , reading and writing.