The website will automatically group your child into the age-appropriate division when you register your player.  The age groups are determined using littleleague.org's age charts. 

You can also use the following link to determine your boy's 'little league' age for baseball. 

Little League Age Determination Calculator - http://www.littleleague.org/leagueofficers/Determine_League_Age.htm

or use the following chart: