someone who went to the DC show posted that there were a ton of kids in the pit so I don't think there is an age restriction, however, that also might depend on your venue.
Yes there are wristbands, it really depends on the night if its crowded or not, a lot of what I'm seeing so far in the US, mind you there's only been a few shows, is that they are slightly packed. I feel like the pit is designed to be a little roomier so that people don't get hurt, but if you have people in there that get crazy, there isn't anything you can really do about it. Again, perhaps you can check with your venue closer to your date.
There is a barrier between the stage and the floor seating, but it doesn't appear to be a HUGE one, but I don't think you can touch the stage from there. There are security guards who basically make sure you stay in your seat (my friend kept trying to get up there but they wouldn't let her)