I didn’t get detailed teachings on it. I just remember being probably 10 or 11 with my dad and we were out camping and there was a little girl with no top on and I remarked on it to my dad and he said it didn’t make any difference when they’re that young. So, I haven’t felt like it was strange when I’ve seen it. I recall one FReeper saying she grew up with the 4 Bs. No Boobs, no butts, no bellies, no backs.
Girls were allowed to go topless playing in the backyard and such but not out in public, up until the age of around 8 or so when I was a kid. But, definitely not if a training bra was in order regardless of age.