So here's my understanding of things ... And someone with experience please chime in if there's more to add!
Past 18 you cannot be kicked out without cause. You cannot be RIF'd, seperated for end-strength numbers, etc.
HOWEVER, you can quit ... Either on purpose or unknowingly.
Based on the chart of doom from the AFI, if you 3-day opt a 365 you have to set a seperation date that is, at most, 12-months in the future.
So if you 3-day opt a 365 at the 18 year point (assuming you have the retainability, TOS wickets, etc to be selected for a 365 in the first place) you are, in effect, unknowingly volunteering to quit at 19 years.
If anyone has any actual experience in this situation, please chime in.