Sorry, you're right about that. It was from...Use of Weapons. Sorry, read them back to back, so I guess I confused them a bit.
I guess we should probably differentiate between the culture level anti-aging stuff, which gives functional immortality, and somewhat lower tech versions, which simply prolong life. The former wouldn't be available till the Culture would be certain that the others could handle it. The latter would be much more likely to be traded.