As I've understood it, quite a few states will let you have a driver's license at the age of 16. Now say that John lives in State A, where you can get a license at the age of 16. Is he allowed to drive in State B while still 16, where you have to be 18 to get a license?
Pretty much, but keep in mind that we've got varying degrees of licenses here in the states.
I know my state of Michigan stopped giving 16 years olds a true driver's license some years back. It's a bit restricted with regards to when you can be driving and the number of other teenage passengers you can have after a certain time. Something like that. I'm not real up to date on the details because it never affected me.
But, once you're 17 you get a full "Operators" license and you should be good to go in any other state that honors the MI "Operators" license which is all of them.
Some states will issue a license for farming purposes to kids as young as 14. I'm quite certain those aren't recognized out of state by anybody. Maybe two bordering states will have an agreement, like we have with CCW laws, but they'd be the exception rather than the rule.
As for myself I have a Michigan "Chauffeur" License. It's slightly harder to get than the standard "Operators" license but not by much. I believe this qualifies me to drive a limo or any other long (non-trailer) vehicle across the US but I never looked into it.
And then we have the CDL (Commercial Driver's License) system that is recognized across the land. This is what you need to drive a semi-truck, school bus, etc. over here. And there are variants of the license too that dictate what you can drive. A trucker can't drive a school bus and a school bus driver can't haul a load of liquid nitrogen. It doesn't work that way unless they have all the endorsements.
Now, I said all that so I could say this: CCW permits work pretty much the same way. The difference is there's no nationally agreed upon gold standard for them, like we have with driver's licenses, and unlike driver's licenses there's only ONE level of CCW license per state. So, we should fix that. In my state (MI) I can carry in something like 37 other states with my license. If there was a Level 2 to that license it'd be easier for the AG to work out a reciprocity deal with the other 11 states that are OK with concealed carry. But only for those people that had the license.
Now, I know it won't be a popular opinion on this board to suggest more licensing with something that we all consider a right but it has worked out well enough for driver's licenses and I think we should try and emulate that. It'd be nice if the federal standard was whatever your state's most common license level was but that's not likely to happen in today's political climate.