Can you name an individual who speaks English as a second language (or any language) with a truly native accent, who has not spent numerous years in an English speaking country before the age of 15 and who does not live in an English speaking environment?
Think about your own native language — can you name a native English speaker who has not lived in your native country as a child and who speaks your language exactly as you would? Maybe you could find one or two individuals who have ‘excellent’ accents, but deep down you can probably tell that they come from a foreign country. If Russian is your native language, then one individual who I can think of who speaks Russian with an near ‘perfect’ Russian accent is Justus Walker, being an American who moved to Russia at the age of 16 and who has lived in Russia for over 20 years, in almost complete isolation away from the English speaking world. Listen to his accent — absolutely amazing, isn’t it? But maybe you can sense a minuscule tinge of foreignness in his voice (or maybe not).
With regards to children and second language accent acquisition, the most likely reason as to why children can pick up an accent is because they have the time to absorb their surroundings and the desire to be accepted by their peers, whereas adults have responsibilities outside of language learning to tend to and, moreover, adult personalities are more calcified, whereas child personalities are in flux and in the process of being formed, meaning that children are more malleable than adults.
If you really desperately wanted to completely bleach away any trace of your native accent, then one potential solution would be, like Justus Walker did with Russian, to isolate yourself away in a community of native English speakers who have your desired accent for at least 8 to 15 years and, of course, you would have to constantly make the effort to speak and listen to your peers for many hours a day. Technically speaking, it is possible to achieve near perfection with your desired accent, but do you really have that much spare time?
As an English teacher, I occasionally have people come to me who dream of speaking with a ‘perfect RP’ or a ‘British’ accent. Of course RP (received pronunciation) and (most) other British accents sound wonderful and I understand the appeal of the British accent.
It doesn't matter where an accent originates from, it is really impressive when someone nails down the fine nuances of the English language and retains their accent. If someone, theoretically speaking, spoke with a perfect English accent and made occasional grammar or lexical mistakes, it would sound almost as if this person were uneducated — native speakers in this hypothetical scenario would just assume that you are a fellow native who perhaps missed out on school.
Unless your dream is to become a voiceover actor or to work as a spy, I really would not focus too much on trying to obtain a certain accent and instead I would focus on correctly stressing words and making your speech as clear and easy to understand as possible.
If your second language were a house, the vocabulary and grammar would be the walls, roof and foundation of the house and your accent would be the flowers in the garden and the pictures on the wall — pictures and flowers are nice and all, but first and foremost you need a house that you can live in, and you don’t want to be worrying about the roof potentially caving in as soon as it starts to rain.
Obtain /əbˈteɪn/ — получать
Absorb /əbˈzɔːb/ — впитывать, усваивать
Bleach away /bliːtʃ/ — убрать
Nail down /neɪl daʊn/ — закрепить (успех)
Retain /rɪˈteɪn/ — сохранять, удерживать
Expand /ɪkˈspænd/ — расширять(ся)
Cave in /keɪv ɪn/ — обрушиться
Numerous /ˈnjuːmərəs/ — многочисленный
Complete /kəmˈpliːt/ — полный, совершенный
Minuscule /ˈmɪnəskjuːl/ — очень маленький
Calcified /ˈkælsɪfæɪd/ — закальцинированный
In flux /ɪn flʌks/ — нестабильный
Malleable /ˈmæliəbl/ — мягкий, податливый
Noticeable /ˈnəʊtɪsəbl/ — заметный
Comprehensible /ˌkɒmprɪˈhensəbl/ — понятный
Foremost /ˈfɔːməʊst/ — выдающийся, главный
Tinge /tɪndʒ/ — оттенок
Foreignness /ˈfɒrɪnnəs/ — иностранное происхождение
Acquisition /ˌækwɪˈzɪʃən/ — усвоение, приобретение
Tapestry /ˈtæpɪstri/ — гобелен
Foundation /faʊnˈdeɪʃən/ — основа, фундамент