Laws that deal with alcohol, public nudity, helmets, seat belts and how fire arms are purchased are all local affairs. Usually these are governed by the states but the counties/cities/towns can add additional laws as long as they don't contridict the state laws.
I think drinking is 21 in all states now but I was able to drink at 18
If a women needs to breast feed she should make some effort to be discreet and the same for changing diapers. I can't imagine someone being arrested for taking care of a child unless it was attracting a lot of attention.