Children know all the common words before they start school. At about age 10, they start learning academic and scientific words (such as sun, star, and planet) in context (such as a simple lesson about astronomy). They hear the teacher and usually use the new words without difficulty.

Babies generally start imitating speech in the second half of their first year. Their first experimental 'words' are usually combinations of the vowel sound 'a' with 'm' 'g' and 'b'. Over the next three years or so, the repetoire of sounds are built up until they have the full range of sounds of their native language. Some phonemes take longer than others to acquire. For example, English-speaking toddlers may say 'f' 'v' or 'd' instead of 'th' for a period. This doesn't mean that they are unaware of the difference between these sounds, however - they can perceive 'th' sounds perfectly, but may need a few extra months to get to grips with saying them. Consonant clusters take longer to learn. An English native speaker of around three years old may speak fluently, but they might struggle over consonant clusters such as sp, sk, tr, gr, gl, and so on. For example, they might say 'tone' instead of 'stone'. Once again, they are fully aware of the difference between 'tone' and 'stone', but just need a little bit more practice in vocalising this difference. Even when a child has managed get their tongue around all the phonemes (usually around 40 - 44 in the case of English), they may still struggle with longer words, missing out syllables or transposing phonemes. By the time children start school at around the age of 5, however they have usually mastered the pronunciation of all the words within their vocabulary. Native speakers struggle with the irregularity of English spelling, but rarely with pronunciation. We all know, from a very early age, how to say the name of the tool for cutting bread, for example. We know how to SAY the word 'knife', but we have to learn how to write it. Occasionally, a native speaker will come across a word written down and be unsure how to pronounce it, but that is rare.

Yes, yes they do. Because English borrows words from many, many, many languages, it can pick up words with sounds that simply aren't common in English. Native speakers (especially little kids) often do have trouble with those. Some of the common mispronunciations aren't even uncommon (e.g., "February")