I usually aim for hitting age up around 8-9 mins (as german), which gives me a fortress age of ~10 mins. I usually make age 2 at around 4:30, and since I focus on gold (I just leave 10 villies on food the whole time, and rarely run out), I can make fortress easily enough. If I'm being raided, I slow down alot, though, usually adds ~3-5 mins to fortress and ~5-10 mins to industrial age.
For me, going fortress is also good for my economy, since most of my free units cards are fortess age (though I have enough to stop rushers). Getting free units lets me build up my econ better, plus I can get plantation/mill upgrades and the last woodcutting upgrade. Note that I also only play team games.
Ironically enough, I usually use the fort card last if at all. I make industrial at around 14-15 mins, and it's easy to get some mortars to knowck down the enemy forts. Then I put mine up, but I use it more for unit production than defense, since they fall so easily in age 4.
When I get level 25 (i'm 24 now, cant wait!), and get the mill->settler wagon card, fortress becomes even more alluring.It really depends on the civ and oyur cards. I have no resource cards in my deck, just the settler wagon ones. No gather rate cards either, I'm better at managing my econ then military so it's easier to get free units and focus on building my econ the hard way.