Since they are elected every four years, the average undoubtedly varies from election to election.
I think the only age qualification is "over 18."
I couldn't find any site that had an average number posted.
There is not anywhere I can find that information. However the people who are electors are people who have established their loyalty to the party they represent. My guess is they would have to be active in their party for some time. When I looked at the list of electors from the last election for my state I recognized only two names on that list. One is 58 now, and the other one I couldn't find an age for but I would guess she was late 45-50 then.
I realize that is only two people, but I figure it takes time to prove party loyalty, and would go with the average being somewhere around 50-55.