Research clearly shows that early childhood is the critical time for music development.  A child in a stimulating music environment receives an aptitude boost.  Where environmental stimulation is lacking, however, children are at significant risk of not achieving the foundation needed to support a lifetime of music learning and enjoyment.

The brain undergoes a period of rapid neural development after birth, continuing for the first years of life.  During this time, new neural connections are forming more rapidly than at any other time in our lives, and during our mid-childhood years, the brain starts to prune these connections, retaining only the most important and most often used ones.  This becomes the basis for our understanding of music and ultimately the basis for what we like in music, what music moves us, and how it moves us.  This is not to say that we can't learn to appreciate new music as adults, but basic structural elements are incorporated into the very wiring of our brains when we listen to music early in our lives.

Daniel Levitin Ph.D. (2006) This Is Your Brain on Music p.107.

The bottom line is that easy and relatively effortless learning and enjoyment of music will be an option for those children whose musical aptitude was bolstered by a rich musical environment in the earliest years of life.