The NSC s a very high level. An NCO can be assigned the NSC. Anyone with the expertise and knowledge can be assigned the NSC.
Why is everyone hung up on low level, high level, being 21 years old etc.
A TS clearance is not dependent on age or “level”.
As I recall, there were some 500 who were “assigned” to the NSC which I find totally ridiculous; Trump’s final appointee to NSC got it down to about 40. A lot of people sitting around doing something close to nothing and having access they neither need nor deserve.