When should I typically start making units?

I have a decent, if a little generic, dark age strategy which I employ nearly every time (with minor variations based on civ) in 1v1 and team games where I'm not in pocket position.

My feudal time is about 11 minutes with 23 pop, okay but not amazing, but I always get thwarted by rushes. Be it a militia drush or scouts or archers (or tower rushes, though that's rarer in DE with towers being nerfed) I just never seem to have enough units, even if my economy is level with or ahead of my opponent's.

I feel like there's some important thing I'm missing that makes me unable to deal with rushes, and can only assume that I'm either not making enough units early enough or possibly not walling properly. It doesn't seem to be a micromanagement issue since my opponent will often have several units in my base before I've even got any. On wide open maps like steppe it can be difficult to wall since there are few choke points that buildings can fill, so defending against feudal aggression is even more difficult.

Should I start making units immediately after a building is up, and try to keep it pumping them out constantly? I usually only have two or three military buildings in the feudal age (depending on whether the starting barracks counts) but even then it's difficult to maintain a constant flow without setting back my economy.

So anyway, when should you start making units, and how do you keep up production?