Here in Portland in 2009-10, about 80% of dancers are single, with 20% being married or otherwise attached. Ages vary from 20-something (or less) to over 70. Median age is probably 30-35.
Attire varies considerably. Some people dress up, while others wear jeans and T-shirts. This is Portland, after all. We tend to be pretty cosmopolitan. In general, practicas are less formal than milongas, and while it's true that fewer people dress up for the Sunday practica than is typical at other events, the Wednesday practica is more like a milonga and a lot of people dress up for it.
The Wednesday practica BTW is an alternative music event, and has the lowest average age of any event currently running in Portland.
Suffice to say that wherever you go, you cannot be over-dressed, nor can you be under-dressed. It's really all about the dancing.
Since most milongas have a beginning lesson as the first hour of the dance, there will be a majority of beginning dancers during the early part of the evening. More experienced dancers tend to arrive from 9:30 to 10:30.