Meh, 'age' should be left solely at the discretion of the RPer. So should time.
In normal writing, the passage of time is a tool that you can use to generate suspense, mystery, etc. (i.e. by fast-forwarding after an event, or having two things go on simultaneously, etc.)
And even in some cases, (like for example 'The Simpsons', though that's not a great one) time stands still (despite over 10 'halloween' episodes).
That's why I feel any kind of 'age' or 'time' kind of restricts the RPing, because there will always be arguements between hardcore players about whether or not backstory or whatnot can fit into the timeline...