I have served as primary preceptor for new RNS on orientation in our nurse residency program. A few of these have been 18 or 19 years old and already RNs.
Our technical college system's ADN RN program is designed, and can be, completed in two years. There are no college class prereqs. Some students have managed to graduate high school at age 16 or 17 and directly enter the technical college's nursing program.
One nurse I precepted graduated high school at 16, completed the ADN RN (cost $6,500) program by 18, and at age 20 had two solid years of ICU experience, a BSN she got online in a year, paid for by the hospital, had earned her CCRN, and made $130k (over two years) by the time she was 20.
She applied to CRNA schools, was rejected, and went travel nursing for two years, then reapplied and was accepted. She graduated as a 26 year old CRNA.
Obviously an exception person, but her path is open to anyone.