Small children learn languages more easily and more quickly than adults do. Starting to learn a foreign language when you are over the age of 18 is infinitely more difficult than if you start as a young child or better yet, a baby. Why is this?
Babies start to pick up on sounds of a language while in the womb. They listen to the “song” of the language(s) being spoken around them, the rhythm, stress, and intonation of speech, which linguists call prosody. If you hum a sentence in English the sound of that is the prosody. If someone then hums a sentence in Spanish, Russian or Chinese the prosody of each language will sound different. Of course, babies that are exposed to more than one language while in utero, and thus more than one prosody, have a head start on being bilingual or even trilingual later in life.