What you saw was a carefully, professionally produced, staged, shot, and edited campaign event that were, in truth,basically little more than the tag end(s) of free rock concerts. Those hyperactive 'yoots' were there for the band(s), not Obama, and melted away once he started to go into his speech. Camera angle. Camera angle. Camera angle.
You said it. It was a big, free party. Great weather and a good chance that your mom would see you on TV. Remember, we invented the CAST OF THOUSANDS! right here in Chicago. Folks my age grew up with it.