Only been playing around a year, myself ... while I'd heard of DF before it was Arstechnica's moaning about the learning curve that prodded me to give it a try.
So I specifically set out to make my first fort a learning fort and played pretty conservatively. Last I touched it, Cerolemal was chugging along at ~15 years in.
Still managed to have some nice misadventures, though!
First ambush wiped out my pathetic military, and my pathetic squad of emergency draftees who gave their lives so that the adequate mechanic could finish linking a lever to the newly constructed bridge across the entrance. At one point a bit later I was turtled for over a year due to back-to-back sieges. Eventually I did discover traps and marksdwarves, though.
Took me a while to figure out building destroyers, too. First encounter was with a minotaur who beelined to my SOOPER SEKRET emergency exit (I guess I thought hiding a hatch in the woods would make it hard to find?). Casualties would have been extreme if not for a hunter who happened to wander upon the scene. Then somehow I got the idea that building a door with only diagonal access would protect it. Mup the FB webber disproved that one.
So Cerolemal never fell, and now it's got magma via a 150-z piston and piles of Mup's FB silk. The foes it can't overcome are FPS and bad layout. I didn't catch on that dwarves move vertically just as fast as between levels, so most of the fort is dug out in a couple of giant layers. On top of that, I somehow goofed up installing the tile pack and wound up genning a world without minecarts or wheelbarrows (but all the slow hauling of the current release, oh yes...).