you need to be a member of USTA to play those tournements (sounds like they are USTA sanctioned events).
Usually it's single elimination. But they have a consolation bracket (a better name for loser's bracket). So you could be the winner among the losers. lol. So you are pretty much guaranteed to play at least 2 matches.
Usually the age are pretty mixed, from old to young. They are all there. IMHO the 3.5 USTA tournment players in MA are more like 4.0 (if you rate skill wise).
When you sign up for USTA, you self rate yourself. Once you've done that, you are only allowed to register for events within + 1/ -0.5 from it (I might be wrong).