I ask because in my first town there were a couple of people with experience, and the rest were almost entirely new. My second town again had a couple of people with experience, and the rest had been in 1-3 towns before.
Is there some mysterious skill assessment going on that ensures the people I'm working with will be steadily more mature? (please say yes)
However, when you join a city, you drag with you as many available people from your coalition as possible -- regardless of their soul score.
This gratuitous formula explains how many soul points you get by surviving for n days in a city:
See the Die2Nite guide for more, especially on the meaning of the term "surviving". There are many corner cases, like death by addiction, that discount your last day.
They're filled up linearly. People who join in the same time-frame therefore come in the same town.
Once you get 100 soul points, you get to join veteran towns, which have a bigger desert and will only have other people with 100 soul points. Therefore, once you have survived for a full 14 days in the same town, you get to join a town with experienced people only.
Of course, when you're a Hero, you get to decide for yourself which town you'd like to join.
So no, they're not secretly skill-matching. They are doing it in a very obvious way.
If you're interested, the formula for soul points is 
where n is the amount of days you lived fully in a particular town. Basically, you add up the amount of days, which means that by day 14 you'll have 1+2+...+14 = 105 soul points.
