Twelve full months of development makes a huge difference for an 11- or 12-year-old. The player who is 12 months older will, on average, be bigger, stronger, and more coordinated than his younger counterpart, not to mention more experienced. And those bigger, better players are the ones given opportunities for further advancement.
The first major study of what has become known as the "relative age effect" was published in the Journal of the Canadian Association for Health, Physical Education, and Recreation in 1985. This study, by R.H. Barnsley, A.H. Thompson, and P.E. Barnsley, determined that NHL players of the early 1980s were more than four times as likely to be born in the first three months of the calendar year as the last three months.
The effect in hockey would probably be even less now because players come so many various countries, all with different youth regulations.