@flowerelf,
Learning and speaking a second language with that qualification of 'pure' is a relative and a loaded term. However, over time and with exposure (immersion) both a native-speaker of English and of Chinese language could get very good. My guess is that even after awhile, there'd still be an accent or certain differences would be detectable but not to the point of being clear communication. Beside the size of working vocabulary, phrases, slang and idioms are very hard to learn about totally.
In my case, being in my '60s, I'd consider learning a language at that level relatively futile. In my early teens, I might've had the facility to do it. Furthermore, understand that little children from age 4-12 yrs of age are amazing at what they can learn as far as second languages.