Are retirees subject to UCMJ?

Those of us who are receiving retirement pay understand that it is not actually retirement, but a retainer for future service, should that be necessary. As for the rest, I'm not a lawyer so I won't argue the main point in your question. It's an interesting discussion though.

Retirees are indeed subject to the UCMJ! Flynn is included IN THIS!!

Yes!

Yes. Often only exercised in cases where offenses occurred while on active duty or where civil authorities can’t reach the misconduct because of some jurisdictional defect, statute of limitations, etc. This authority is used VERY rarely and requires Secretarial approval.

Unfortunetly, this is a fact. We retirees are still subject to punishment under the UCMJ. If you want to know which articles still apply to retire persons,contàct the cĺòsess base JAG office.

On 22FEB2019, The United States Supreme upheld The Department of Defense's authority to allowed
retired military service members to be Court Martial. The U.S.S.C. denied a retired Marine SSG to petition the court for criminal activity conducted after the individual was retired. Case was Larrabee v. United States.

In my experience with military legal, typically the only time the military will "reach out" & grab a retiree is when a serious violation of the UCMJ is discovered to have taken place during the time that the retiree was on active duty, and that the retiree meets all the criteria for indictment under an Article 32 investigation. A 15-6 investigation can be used as an initiating circumstance/triggering event, to warrant an Article 32 investigation. Most other crimes are typically handled by appropriate city, state or federal authority depending on the nature of said crime and that it was not committed while the retiree was on active duty. The 1 exception is if the nature of the crime is something so serious so as to bring discredit upon the service as a whole. Such a crime would likely be pursued by all levels including active component service branch, such as rape committed by Marines on Okinawa which caused the locals to push for the Marine base to be removed from the island.

Personally, I have no problem with being under the UCMJ for life even though I'm retired, as I had no problem being under it while on active duty. I signed up in the USCG and never claimed to say I now consider myself no longer my oath when I enlisted. I am a Coastie for life, and If I am guilty of something covered by the UCMJ, I should be punished for it.

...of a regular component. Does that include members retired from a Reserve Component?

Talked to a JAG Lawyer. There are 3 circumstances where Retirees (and even non-retired vets) can face UCMJ:
1. For any crime committed in service where the final disposition was never made, even if the service member left the service 100 years ago. This is for both veterans AND retirees and for both misdemeanors and felonies.
2. If a service is receiving pay FROM THE BRANCH OF SERVICE and commits a felony. This does not extend to VA paid disability. Does not apply to misdemeanour. For example, if a disabled vet/retiree chooses Army/Navy/whatever compensation in a lump sum instead of VA percentage, then they can be tried under UCMJ. The VA has their own policies for handling vets who commit felonies and those policies are backed by federal law, not UCMJ.
3. The obvious one, desertion. I faced this one because I was admin separated from the Navy with an Honorable Discharge but RE4 reenlistment code due to Captain's Mast. A year later I was arrested by local law enforcement and turned over to NCIS. It was just a mixup of paperwork and got sorted out just fine, but it prompted me to get a lawyer. What prompted it was I had orders to FMTB but they never got the memo of my separation and put out a warrant for my arrest lol.

Side Note: While Article 2 does not mention nonretired vets, it has been confirmed to apply to them. However compensation is so rare for nonretired vets. The GI Bill is a VA Benefit, not a military one. The military has no control over the GI Bill except for what discharge they give, which affects eligibility. So a vet who only receives GI Bill and VA Home Loan do not fall under article 2.