I played the game for a while and then stopped. If you want your people to be as powerful as possible, I think you have to get their agility to 10 before having them level at all. Otherwise, they'll have less HP than they otherwise would. By having more HP they can survive longer against the monsters.

So you have to work until you get the A-training room, then make babies that you can send there to train. Once you get them trained up, you want to send the already leveled people away to make room for more babies that you can get to 10 agility before leveling them. It takes a long time but it makes them stronger.

By the time I realized the strategy, I had already put a lot of time into the game and didn't want to just exile all my people. I ended up quitting shortly thereafter.