in study published in 2001, Linda Polka and colleagues found that at 6-8 months, babies exposed mainly to French and those exposed mainly to English could both distinguish words like "doze" and "those". (They differ only in the initial interdental.)  At 10-12 months, English-learning babies had already begun to pull away from the babies learning French.  By adulthood, English-speakers are significantly better at perceiving the difference between the initial sounds in "those" and "doze".

Language learning: Babies and sound perception | The Economist Wednesday, October 16, 2013 @ 8:28pm