As piercing is for life, I think it should be the child's choice, not the parents, and probably not before around 12 yrs.

Dummies - depends on the child, but ideally not over the age of 1 yr if possible, as use can damage/delay the development of speech.

Pushchair - up to 3 yrs.  Children need to walk, and they need to learn how to take responsibility for staying with their parent/carer in a crowd, etc.

BB gun - undesirable, and there are plenty of other weapons that can be played with that don't have the potential to damage others in the way a BB gun does.

Smoking - undesirable, but realistically a parent who smokes is going to do so at home.  Outside should be the norm.

Home alone - from 12, depending on the child's maturity.  If with other children, one should be at least 14.  No child under 16 should be left alone overnight, IMV.

Alcohol - a small amount within the context of family meals or occasions.  Not under 10 (older if the child is underweight or of a slight build) and very dilute till 14 or so.