This is what I found from IRS webpage:

 From Publication 929:

Earned Income Only

A dependent who has only earned income must file a return if the total is more than the amount listed in the following table.

Marital Status	Amount
Single
Under 65 and not blind		$5,800
Either 65 or older or blind		$7,250
65 or older and blind		$8,700
Married*
Under 65 and not blind		$5,800
Either 65 or older or blind		$6,950
65 or older and blind		$8,100
*If a dependent's spouse itemizes deductions on a separate return, the dependent must file a return if the dependent has $5 or more of gross income (earned and/or unearned).

Example.

William is 16. His mother claims an exemption for him on her income tax return. He worked part time on weekends during the school year and full time during the summer. He earned $5,900 in wages. He did not have any unearned income.

He must file a tax return because he has earned income only and his total income is more than $5,800. If he were blind, he would not have to file a return because his total income is not more than $7,250.