Sequential bilinguals, those who learned a second language later in life, almost always retain a perceptible accent, no matter how proficient they are in all other aspects of a language.
In point of fact the three actions of perceiving, determining, and responding were sequential; but so infinitesimal were the intervals of time between them that they appeared simultaneous.
In sequential testimony, each one stated that he did not believe tobacco was a health risk and that his company had taken no steps to manipulate the levels of nicotine in its cigarettes.
Simultaneous bilinguals learn both languages at the same time in infancy while sequentials learn their dominant language first and a second later in life.
Unlike the cognitive advantages, these are available to simultaneous and sequential bilinguals, so it's never too late to benefit from learning a second language.
While a sequential bilingual may maintain a similar level of proficiency, there are fundamental differences in how these languages are acquired and used.