If you have never had a US passport, if your passport was issued over 15 years ago, or if your expiring passport was issued when you were under 16, you must apply in person. Passports issued to children under age 16 are valid for 5 years. Children under 16 must renew their passports in person and follow the same guidelines as when the passport was first issued, including original birth documents and being accompanied by parents or guardians.
If your passport was issued when you were over 16, and it has not yet expired or has expired within the last five years, you may be able to renew directly by mail. See travel.state.gov for details.