Apply now. Most jobs say 18, but there are some where you may work part-time and after-hours while attending school. Also, when you say package handler, do you mean manufacturing, retail, or warehouse? Manufacturing and warehouse may stick to specific shifts, while retail may offer a flexible work schedule.

If you're underage, you may need a work permit from your school district (it's required in California), so ask your school counselor. As an underage worker, your state of residence may allow you to work weekdays through certain hours, not exceeding so many a week, and no later than a specific time. State law may be flexible on weekends, within the so many hours a day but no later than a given time. However, you may not be allowed to work more than so many hours per week; and if your grades drop below a certain grade point average, the school district has full authority to revoke your work permit. It may sound draconic to a freedom-loving teenager (hey, I wasn't born old), but your principal job is student.

No matter what, apply hard to work hard, study smarter, and succeed.