DoD 5200.2-R -- C.3.3.2.1 An ENTNAC shall be conducted on each enlisted member of the Armed Forces at the time of initial entry into the service. A DNACI shall be conducted on each commissioned officer, except as permitted by paragraph C3.3.4. of this section, warrant officer, cadet, midshipman, and Reserve Officers Training Candidate, at the time of appointment.
C3.4.2.2.1. shows that a DNACI or ENTNAC is the investigation required for a final SECRET.
Granting access to classified information is, granted, considerably different and is based upon the person's actual job assignment and need to know. Thus actually having access is considerably rarer. But the investigation is done and adjudicated on all first-term enlistees as well as new appointees