From observation of myself and those around me, I'd say that multilingual adults have an advantage. They are more used to the process of learning a new language. Learning a new language, especially one not closely related to ones you already know, involves letting go of many "reference points".

Conversely, monolingual adults (or even bilingual ones) have it harder exactly because of those "reference points" that they try to match to the new language. That's not how it works.