Can a now-retired (as of today) Senior NCO be "charged" with prior fraternization with a junior enlisted under his command after the fact?

I have seen Commands bring personnel back to face a Court Marshall, but not for minor offenses. The offenses were Felonies and had a high probability of changing the characterization of the previous Discharge. In one case, a Nurse in my Command, was a Reservist doing an 18 month tour on active duty, she stolen narcotics on multiple occassions for her own use...The Chief of Staff was pending charges and the Brigade S1 let her REFRAD even though she was pending charges...The Commander was able to get the Reserve Command to issue her new orders returning her to active duty...this took at least 9 months...When she returned to our Command, She was arrested and faced a Court Martial..was guilty and ended up with a Chapter 10 Dishonorable Discharge AND her RN was revoked.

Yes. A retiree is still subject to courts-martial jurisdiction. However, it is very rare that a retiree would be bought before a courts-martial. AR 27-10 states "Department of the Army policy provides that retired Soldiers subject to the UCMJ will not be tried for any offense by any courts-martial unless extraordinary circumstances are present." The other branches make a point to note that this includes referral for trial by courts-martial for violation that occurred while the retiree was on active duty. Essentially, if they were going to be charged it should have been done before they retired.

Can they? Yes. Will they? Another story.