I do not agree. I met many peoplke who learnt another language as an adult and they spoke like a native. No accent at all. One of them was an american, When he told me that he stayed in Germany only for 3 years and that he was american and that, didn't study german before arriving here... i was really stunned. He spoke like a native, 100%.
I know a french guy who lost his accent completely, a german women who speaks indonesian like a native (learnt it as an adult as well).. there are many examples.