Storminator
First Post
Settings can't go out 'out of date'. They can, however, accrue a large body of uninteresting ...
I would say that when previously interesting material is no longer interesting (the older set has played it out, and the younger set doesn't care...) then the setting is out of date.
I think it's difficult for a popular setting to go out of date, because it was likely popular because it's material had some strong universal appeal. I think a quirky, interesting, marginal setting could go out of date. Spelljammer? Planescape? Maybe some parts of Eberron will go flat (if you tire of pulp and noir (ahem: HERETIC!) what else would you play there?) if left untended.
PS