Well, of course she wouldn't; she couldn't escape nearly as fast with that parachute attached to her dome.
I kid, I kid.
Regarding the Norwegians; 19 does seem young for SF. AFAIK, it's currently possible in the US MIL. Of course, it's incredibly unlikely.
I can't speak for the Marines or Air Force, but the Navy has a program wherein a 17 year old can enlist under a SEAL contract. Theoretically, at least, one could earn their Trident before their 20th birthday; I'm unaware if anyone has done so.
As for the Army; there is a similar program-- a Special Forces contract for new recruits-- but as of 2013, that was closed to anyone under 20. (May not be the case now).
The other, more traditional, route would be to enlist on a non SF contract, and just drop an SF request once you got to your first unit. But the problem there is, almost noone is going to approve that request if it's coming from some 17 1/2 year old kid who just got out of Basic. We were almost perpetually under strength, for one thing, and it was always a crapshoot on whether the FNG's would turn out to be dirtbags or decent soldiers, much less viable SF candidates.