[stuxnet79]

Accent is fluid and can change when you are young but by the time you are 15/16 it becomes difficult to "lose" your native accent regardless of the amount of exposure you have to a different language. This has been scientifically validated (can't find any relevant papers at the moment to link you to though).

[TulliusCicero]

He's not talking about losing it completely though. Some people are able to mostly lose their accent and sound relatively close to a native, others retain a very strong accent despite having lived in a new country for many years.