i'm in the medieval age with only a few defensive upgrades left.
i'd recommend rushing to classical and then staying there and maxing that. farming was shockingly easy there, so i'd cap everything.
i don't expect to finish all my library research at medieval. instead, i'm planning on staying medieval until i've upgraded all my walls (at least to the 90k level, possibly some to the 200k level) and finished all the critical library research (extra garrison, for example). there are, however, lots of library upgrades i don't care about (and probably never will). i'll upgrade all the food based library research until i'm done with defenses and walls.
that's my current plan, at least.