The average age for toilet training in the United States is currently between two and three years. This is why children trained earlier are frequently considered "advanced" (though there is no relation between age of toilet training and a child's intelligence) and those not trained by age four may be looked down upon.
While no child should be pressured to do something just for the purpose of keeping up with his or her peers, your child may become aware of others' disapproval, which can have a negative effect on his or her self-esteem.
If you see signs that your child is beginning to feel bad about himself or herself, you may want to initiate the training even before he or she expresses any interest. Chances are he or she is ready and may just be waiting for more active encouragement from you.
Additional Information from HealthyChildren.org: