If your teenager was under 16 at the end of 2018 the return needs to be filed by mail. It goes by their age in tax year 2018.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS and/or your state received the return.