As old as you want to make it? I'm not sure why this (civ age) needs to be defined any more than in just a general sense. Especially before anything specific has been developed in terms of... well anything. It's kind of like deciding what colours are going to be in your painting, instead of deciding what the subject itself is going to be. If you like "modern" art garbage, than I guess that is a valid approach, but it doesn't really end up with anything worth while. Only in this case it's not just limited to one's own work it will also bind everyone else's too. If we develop more specific things area by area it will develop more naturally. So instead of just saying "there are Vikings in this world" which is essentially meaningless one could make a Viking country in a plot or one could develop a Viking culture and place it on a plot, and then people who come by later will say oh look there are Vikings on this continent I'll account for that in my map or story. The difference is that they are not just ideas of Vikings but a specifically developed and defined Viking. Holy crap, I'm not making any sense...

I think the approach to take regarding civilizations and races etc, is the same we take with maps. Come up with a specific civ/race idea make a development (wip) thread and start writing it down. If you know where you'd like to put it in the world, claim that plot and attach the information to it. If not just leave it in the air to let mappers and story tellers to grab it as they will. But I think specificity regarding these topics is as important as it is for the mapping part.