Arnold's been acting in American movies since 1969 (Hercules In New York). Why does he still have such a strong Austrian accent?
He hired a voice coach but his agent advised against it. It became his trademark so there was no reason to stop.
Nowadays, he employs a voice coach in order to keep the accent because it's part of his image.
Supposedly, if you move after you're twelve years old, you'll always keep your accent. He left Austria when he was in his mid-twenties, if I recall correctly, and by then, your accent is pretty much going to stay with you all your life. It might get worn down, but for most it'll always be there, and noticable.