I looked today to see what year the actor who played him died....only to find him still alive-tuns out that he was only 41 or so in '67 and is now 80. Must have been quite a makeup job.
Very ironically, the actor who played young Mordred is deceased
Lionel Jeffries is one of those "lucky" character actors who can pull such
a thing off. He played Dick VanDyke's father in Chitty Chitty Bang Bang, despite the fact he was younger than VanDyke by a year!
Other "lucky" actors include Donald Pleasance, Walter Brennan and Ian McDiarmid who could convincingly play "old" people for years despite their relative youth.