It's really the quantum. Five grams for a young person will expose them to criminal liability, and for an adult it's 30 grams. An adult who is arguably more morally culpable is entitled to possess more; it sort of takes them further from the reach of the law. Admittedly, a young person would be prosecuted under the YCJA, but those are still real consequences for a young person, and it's important to remember that.
