Researchers have found that children under 12 can change languages and accents very easily.
With people much older than 16, any change in language and accent becomes much harder and gets increaslingly more difficult as you age.
I don't think you should aim for a native speaker's accent unless you live in an area where everybody speaks that language and will speak it to you. You would have to speak it every day, throughout the day for about one to two years.
You will be much more realistic if you concentrate on making yourself understood - getting enough of an accent that people won't misunderstand you, especially with basic concepts.