Steven H Silver offers us this Graphicle (as well as the notes below) of Worldcon Guests of Honor by Age:

The chart includes ages for 170 Guests of Honor whose ages at the time of their GoH-shop were known. 13 GoHs are not included.
Individuals who were GoHs multiple times (Brian W. Aldiss, Robert Bloch, John W. Campbell, Frank Kelly Freas, Robert A. Heinlein, Fritz Leiber, Clifford D. Simak, Bob Tucker, and Michael Whelan) are included at their ages for each of their GoH-ships. Campbell and Heinlein were GoHs three times. The rest of the repeaters were GoHs twice. Takumi Shibano, who was a GoH twice, is not included as I was not able to find his age.
The average age is 55.6.
The youngest GoH, at 27 was Susan Wood in 1975.
The oldest GoHs to attend were Jack Speer and William Tenn, at 84, both in 2004.
Frankie Thomas would have been 85 and Elsie Wollheim 86, but both died prior to the Worldcon at which they were being honored. Charles N. Brown, who would have been 74 in 2011 where he is a GoH, died before the announcement of his Guest of Honorship was made.