I tried to answer that in my last reply to you above, but let me try it again.

First of all, if Sidious is right about there being an absolute max age when a channeler is bound by 3 oaths, that age is probably not exactly the same for everyone. Stength in the OP still probably plays a role in what that max age is - the stronger you are, the greater the max age, in general (I guess). That's why Cadsuane hasn't died yet, but other AS have died at ages less than hers.

Let's say Romanda is 270 (do we know how old she is exactly?). And, Let's say her max "3-oath-bound" age (going with Sidious' theory) is 290. Before she unswore and reswore, she had 20 years to live. After she unswore and reswore, she still has 20 years to live. Nothing changed, because she hasn't reached her max "3-oath-bound" age yet.

Now, let's say Mesaana is 350, and her max "3-oath-bound" age is 320. If she swears 3 oaths, she would die, since her current age is greater than her max "3-oath-bound" age.

That's what Sidious is saying.

Does that help?