It comes down to more than just earned income when determining if you need to file taxes. Other circumstances include: age, if your income was earned from social security, if anyone can claim you as a dependent and your filing status.
In some cases even if your income is under the required amount it is in your best interest to still file. If, for example, you had a part time job for a portion of the year and you had federal taxes withheld, you could be eligible for a refund. If you don’t file then you will never see this money.
For more information about filing your taxes visit http://www.irs.gov/publications/p17/ch01.html#d0e5173