If it concerns Catholics..

This is the age at which a child is old enough to understand the moral consequences of his or her actions and can be held accountable for sins. It is also called the 'age of reason.' Though it does not correspond to a particular age for every person, due to differences in personal and psychological maturation, it is sometimes set down arbitrarily as 12 or, in the Roman Catholic Church, 7; http://en.wikipedia.org/wiki/Coming_of_age#Age_of_accountabi...

Pelo fato de somente adultos ou crianças maiores poderem se converter, o batismo é negado às crianças que ainda não alcançaram a "idade da razão" (em torno dos sete anos).
http://www.veritatis.com.br/print/1911