As infants, deficiencies aside, we are all born with the same ability to remember and mimmick sounds we hear around us. An infant's mind is like an empty canvas that will quickly be painted with all sorts of twists and tricks of the tongue.
When we are this young, we are all able to learn any language equally. As we grow, our minds grow as well and in order to hold the massive amounts of sounds we hear often, some other not so familiar sounds are quickly forgotten. While a native English speaker remembers the sound "th" pretty darn quickly, a native Spanish speaker may have more proclivity towards "nyo".
As our minds develop and try to expand upon more knowledge, the ability to recognize and quickly process sounds that are not native to us are lost very rapidly. We remember sounds we have heard over and over and can quickly stream them together to form thoughts and words, however sounds that are foriegn to our ears force our minds to slow down and process the incoming information more slowly. Because our minds at a younger age are rapidly learning things, we inadvertently learn to ignore these sounds.
This explains why it is easier to pick up on another language while we are young versus when we are older. This also explains how one can grow up learning two different languages fluently.
(Learning to differenciate between the two languages is a skill learned later by the child, however.
Sorry, I often study linguistics and the psychology of such on the human brain. It is actually quite interesting to see just why our brains don't like to try and process strings of sounds that we don't recognize. xD
Now, I've heard English is harder to completely master as a foreign language. Mostly due to the many lingual influences that created English, the pronunciation of many words may not seem to follow any sort of universal rule or system. It may seem random to those that are not native to the language. That, mixed with our wide abundance of sharp and loose sounds as well as our sometimes crazy grammatical rules, makes English a pretty tough little cookie to crack. xD