Is having your 1st baby when you are young give you prevention against breast cancer?

It may. Studies have shown that women who have children at an early age have lower breast cancer risks. For example, women who have their first child by age 20 have half the risk of estrogen-sensitive breast ca as a woman who had her first child after age 30. This risk may be further decreased by breastfeeding. While we don't know the mechanism, we assume it has to do with less estrogen exposure.
Studies suggest yes. Epidemiologic data suggests yes, but in most societies, having children in the late teens/early 20s is not the norm.
Probably. Having a baby and breast feeding provide some protection against breast cancer. However, this should not be a reason to have a baby.