Each carrier has their own age policy if you are under 18 and wish to travel on your own, or if you are a parent purchasing a ticket for your child, we suggest that you call the carrier directly to discuss their age policy before purchasing any tickets as most are non-refundable.
Most carriers do not allow minors under 17 years old to travel alone, overnight, across different states or on trips that require transfers. Since valid, state issued IDs are required for boarding, some carriers will require that underage traveler tickets be purchased in person, at the station or directly over the phone by the child's legal guardian or parent. Other's may require a signed parental form.
Here’s a list of the contact information for every carrier available through Wanderu: https://www.wanderu.com/blog/bus-train-carriers-contacts/