Duncan MacLeod, 5000 Year Old Immortal

Swap the ages of Duncan and Methos, changing as little of their personalities as possible in the process. What experiences make them act the way they do, despite their drastically changed ages (ie, why does Duncan still fight for justice and whatnot, and let his female enemies live, etc.)?

If doing this as part of canon, not an AU (well, it's an AU anyway, but I mean with everything outwardly the same), Methos Adam recognizes Duncan immediately, and they play out the charade of Adam being Methos.

All ideas are free for the taking.