From IRS publication 501: A single dependent must file if any of the following apply
(a) unearned income was more than $850,
(b) earned income was more than $5,150, or
(c) gross income was more than the larger of $850, or earned income (up to $4,850) plus $300.
Unearned income includes taxable interest, ordinary dividends, and capital gain distributions. Earned income includes wages, tips, and taxable scholarship and fellowship grants. Gross income is the total of your unearned and earned income.
If the teen paid federal withholding, he or she should file anyway to get a refund.
Publication 501:
http://www.irs.gov/publications/p501/index.html
If part of your earned income is from tips, see Tax Topic 402, Tips:
http://www.irs.gov/taxtopics/tc402.html