Jim Rogers frequently mentions that he has hired a Chinese nanny to ensure his daughter grows up speaking fluent Chinese. Studies show that children stand a better chance reaching fluency in a foreign language than those who start learning a new language after reaching adulthood. Adults, starting from scratch to learn Chinese, face a number of difficulties.
First, Chinese is a tonal language with the standard Mandarin Chinese having four tones. The fact that Cantonese speakers cannot agree on the number of tones in Cantonese hints at the difficulties a tonal language might pose to someone unaccustomed to using tones. For instance, the Chinese word that sounds like ‘ma’, can mean ‘mother’, ‘horse’, or ‘to scold’, depending on the tone. A second difficulty arises when one tries to learn to read or write Chinese. In contrast to the 26 letter alphabet used to construct words in English, every Chinese word is written differently. As a result, around 3,000 Chinese characters must be set to memory before one can attempt to read a newspaper. Finally, Chinese frequently use idioms when communicating. These phrases are usually only a couple of words in length (four characters is considered a ‘standard’ length) but learning all the idioms in common usage would take some time. To understand usage of an idiom, one would need to have familiarity with the fable supporting the idiom. For more on idioms, here is a Wikipedia entry on four character idioms and here is a link to examples of the fables supporting Chinese idioms.
For adults who want to learn Chinese, there are some compensating factors. Chinese grammar is generally pretty straightforward. No declensions need to be memorized, as in many European languages, and there are no special words for past and future tense. Perhaps the best compensating factor is that most native Chinese speakers will enthusiastically compliment any attempt by a non-native speaker to speak Chinese, no matter how butchered the pronunciation.