At that age the tots' minds are developed enough to learn basic language skills but are not sufficiently tuned in to one language ahead of another.
Researchers at Paris Descartes University suggested that 20 months is the age at which children in bilingual homes begin to become proficient in both tongues, reports The Daily Express.
In their study, boffins were trying to get French children to understand simple instructions in English.
All of the 24 youngsters came from homes where only French was spoken, said the team.
The instructions, such as telling them to put a ball in a cup, included words they had yet to learn in French as well as English.
The results, published in the journal Infant Behaviour and Development, showed the babies picking up the English as quickly as the French instructions.
Researchers said the study showed that "20-month-olds have developed mature word learning mechanisms".