Sorry for the lengthy post, but this is one of those things I found very interesting and it has implications for anyone with kids.

I subscribe to a baseball-oriented blog that is usually full of arcane stuff like does a reliever's win/loss record means anything or should Alan Trammell be in the Hall of Fame.  But recently there was an article about birthdays and the impact it has on a person's success.  Here's a small chunk of the article:

"In a chapter of the 2008 non-fiction book Outliers, author Malcolm Gladwell studies the birth dates of successful hockey players, discovering that a disproportionate number were born towards the beginning of the calendar year.  Gladwell's explanation for this phenomenon (termed the "Matthew Effect") relates to the age cutoff date for youth hockey leagues.  The Canadian author points out that the nearly universal cutoff date for hockey leagues is December 31.  Players born shortly after the cutoff date are a few months older and more developed than their average competition and would therefore have a slight advantage."

"However, when the slightly older, bigger, and stronger athletes are selected for All-Star teams and travel squads, they spend more time honing their skills and benefit from superior coaching and the added attention.  Within a few years, the slight age advantage has turned into a significant ability gap - not because the players born in January or February were naturally better athletes than the December birthdays but because they had greater opportunities to develop."

"Gladwell notes that we see evidence of the Matthew Effect elsewhere. The American education system is largely based on arbitrary cutoff dates; in Alabama, for example, the cutoff date for most schools is September 1.  A child born on August 31, 2000 would start kindergarten in August 2005, but a child born one day later would wait an additional year to begin his/her education.  Cutoff dates vary from school to school and state to state, most falling between August 1 and December 31.  Regardless of when the cutoff date falls, Gladwell's theory holds water - the oldest children in each grade hold an inherent advantage that does not simply disappear with time.  In fact, Gladwell says that children on the younger end of their grade are underrepresented in colleges and universities by over 10%."

The article then goes on to evaluate baseball players.  For 55 years, Little League had a cutoff date of July 31st.  And as it turns out, there have been a disproportionately large number of major league baseball players with Aug & Sept birthdays, and a disproportionately small number of major leaguers born in June & July.  It isn't a huge difference, but I didn't realize it would matter at all.

So when (during the year) you were born can cause you to be grouped with others who are generally younger than you or others who are generally older than you.  Which group you end up in can make a difference in what you go on to accomplish.  If you're the oldest kid in your group/grade/league, you'll generally be better than others and thus be given more positive feedback and more opportunities.  You'll more often be successful and that probably contributes to future success.  Conversely, if you are the youngest in your group and everyone else is better than you at something, you're probably less likely to pursue it, even though you may actually be good at it.  It's an argument for putting your kid into a position to succeed rather than into a difficult situation, in the hope of challenging them.