1. Depends on preferences. The younger a world is the more beasts there will be, the older a world is the more towns, bandits, and artifacts there will be. I go for somewhere between 200-400 years to get good amounts of beasts and good amounts of towns. Though I also set beasts and civilizations to pretty high levels.
2. You can if you want, but you don't have to. I typically find visiting fortresses as an adventurer kind of underwhelming, because most of my designs are rather simple and the game just assigns people to positions randomly so suddenly my legendary mason is the sheriff? But maybe if you specifically design it to be fun for an adventurer it could be nice.
3. Yes. Yes. Yes.
4. I'd definitely suggest doing quests over random wandering. You're not going to find anything interesting to kill just wandering around, and there won't be many good stat improvements from stabbing deer. If you ever find yourself needing stats, you can find yourself a bridge across a river in a town and get your swimming skill to at least competent, and then swim under the bridge. This will cause you to start drowning, which is dangerous but at standard movement speeds you should easily be able to survive 8 movements while drowning. Now why do you want to drown? Because it very, very quickly raises your endurance and toughness.