Although anyone under the age of 16 technically must follow the unaccompanied minor guidelines, it is almost impossible for them to enforce. The Amtrak website states that anyone under the age of 18 does not need ID on the train, and even when checking luggage a student ID would work, which often do not have the person's birthday on it. As long as someone looks close to 16 and does not cause any problems, it is unlikely to be an issue. I travelled on 6 Amtrak trains alone before I turned 16, and for only one of them I rode as an unaccompanied minor (Tampa-Ft. Lauderdale). I rode Baltimore-Atlanta, Atlanta-Charlotte, Charlotte-Raleigh, Raleigh-Tampa, and the Autumn Express all at 15 and it was never an issue. Once I realized that I could do that, I would not have travelled as an unaccompanied minor even if it was allowed on the route. The one time I did was one of my less enjoyable Amtrak rides as I wasn't really even permitted to walk around the train freely. Travelling as an adult also allowed me to connect to local transit without anyone picking me up, which worked well in Baltimore (MTA Maryland), Atlanta (MARTA), and New York (LIRR/Subway).
I also travelled on Southwest unaccompanied multiple times before I started travelling alone on Amtrak, as their required age is only 12 or 13. On Southwest, I went roundtrip Tampa-Chicago, and a loop trip Tampa-Washington-Atlanta-Tampa. This also did not require someone to pick me up, although someone did in every city except Atlanta, where I used MARTA.