I know this has been a hot point of debate and I'll admit I didn't slog through all of it so forgive me if someone has already argued this point with you.

I think when you take the oaths, they cut whatever remaining lifespan you would have had roughly in half. The WT Aes Sedai don't live much past 300, but one and all they swear the oaths very early in life. So their projected remaining life was approx 600 years, they get only 300. If Mesanna has lived about 300 years, and then took oaths, I think they would take her remaining 300-400 years and cut them in half.

She would not appear to age on the spot, or die, or appear ageless. (note: I think agelessness comes from time spent under the Oaths, not age + oaths) So Mesaana would appear exactly the same, but her projected lifespan would be only 150 or so more years. After 20 years she would look ageless just like an Aes Sedai. But none of that last bit would happen if she removed them quickly.

I am NOT saying I think she took the oaths and removed them, I'm just saying if she DID...