I'm probably not the best person to field this question given that I was not around for a number of the transitions between ages, but I'll try.
Historically, it looks like a "new age" was often used to make changes too large in scale to simply reboot the MUD and be done with it. This might include major code changes, heavy change-ups in cabals, or major changes in policy.
And then of course there is our current age which is the result of a hosting snafu and was not deliberate.
I wouldn't hold my breath for a new age any time soon. The short version being that there simply is not likely to be a need either from a IC narrative standpoint or a technical one, that I know of, in the near future.