In my case, as I can't speak for others:
The directives from US Soccer asks referees to manage the game in a way that allows the game to flow with only necessary stoppages of play. I am one of those "loose" referees tending to find careless fouls "trifling" whenever possible if the foul doesn't affect gameplay (i.e. offensive player not affected by jersey pull). The caveat I make is boys of the same level and age tend to be called looser than girls of the same level and age BECAUSE (1) parents of girls are less tolerant of their DD falling to the ground and (2) girls are more prone to injury after puberty compared to boys of the same age.
Basically, I agree -- Age and Level.
Loosest: Boys/Girls - Flight 1 / Premier - they generally have good control of their bodies, can anticipate strong (fair) challenges. They also have a tendency to strategically needle their opponents into fouling them, but generally, respect their opponents.
Loose: Boys: Silver/Gold/Flight 2 - at U16/15 they are becoming men ... still a little gangly and full of misdirected testosterone. This group is actually one of the toughest because these little bastards think its OK to start swinging ... dumbasses.
Little Tighter: Girls Silver/Gold/Flight 2 - because parents are more sensitive and get sensitive when a strong challenge is made. These girls have a potential future. More so than the boys, but we now have some parents that are much more vocal (f'ing parents, please be quiet). These parents are the most likely to view this game through their rose-colored glasses.
Looser than REC: U13+ Boys/Girls Flight 3 - basically glorified REC players with better skills. Parents that don't understand that soccer is a contact sport.
Normal - U11-U12 - Boys are looser and girls are tighter. The parents are proud ("my kid play club soccer ... flight whatever"). This is an easy game because the kids are not A-holes. Only the coaches and parents. The kids are not reckless, just careless. My only issue with these kids is telling the mid-fielder to stop listening to his Dad that is tell him to "not take it from number ??? and push him back." God, I wished this Dad wearing his Raiders cap would shut the f up ... and what is with the neck tattoo on your dad?
Back to Loose 7v7 - Just play. These little kids don't have any malice in their heart. I leave my yellow and red card in its case. Just play, but stop listening to your Dad telling you to "take it to goal" and listen to your coach.
Tight - Rec.