You need a captain of the guard or mayor to join a fortress fortress when you retire, so I think that means the population as to be at 50 (if I remember correctly). If you simply retire your adventurer at the fortress the *may* also join of their own accord (I've seen it happen), but it is not guaranteed.
I've played a game where I got a captain of the guard and then I made an adventurer who travelled from the mountain home in order to join the fortress. After joining the fortress (if you want to be a guard, you need to make some notable kills, BTW), I had him training for most of the time. A year later I retired the fortress and had him adventure again. The cool thing about this is that because he had a good reputation before joining the fortress, he was able to recruit people *in the fortress* to join him (only people who are on active duty and not nobles will join, so put the people you are interested in into a squad and have them be training before you retire the fortress). Unfortunately, my adventurer died (somehow goblins got hold of some steel equipment... I'm still not sure how). I then started a new adventurer from the fortress and immediately retired (well, she was in a tree, so I got her out of the tree first). Because she was a citizen already, she was a member of the fortress when I unretired. I put her to training again. Unfortunately she died in a goblin siege. I did it a couple more times, but eventually got bored.
I've been meaning to do it in the new version, but haven't had time to play at all. The most cool thing about doing this was that I was able to be really involved in the outside history. Because I sent out an adventurer every year, I could go to towns and see the aftermath of armies, etc, etc. I was even able to defeat the leader of one army (albeit by accident -- I noticed a goblin in a destroyed town. I walked up to him and asked his profession. My companion cut off his head before he could answer. Later I noticed in legends mode that he had led the raid on the town.) I think this would be even more fun mixing it up with raids and searches for artifacts.
Anyway, it's super fun. I highly recommend it -- and if you like fortress mode, it's an easy way to avoid grinding adventurer stats -- just play the game normally and allow them to train. The problem, of course, is living with the fortress unretire problems. The main problems are endless random merchants, goblins, etc joining your fortress; items that are unmoveable and unownable (usually you can clear the status if you try to build something on the spot and the dwarf moves the item in the process); clearing all your manager tasks; all your items being removed from their containers (including alcohol! make sure you have a way to easily kickstart brewing); beds not working unless you completely reassign all of them; and tens or hundreds of visitors who are marked "hostile", but are anything but (mostly you just have RP an excuse to kill them off -- eventually I had a pet were beast that I set loose to aggro/kill everybody... it was "fun").