At what age can a minor child choose which parent to live with in a divorce? And does the parent the child chooses then get primary custody?

Presuming it is a Nevada order, there IS no "magic age" at which a child can choose, but some courts recognize "teen-age discretion".  For a more complete explanation, see the materials posted at http://willicklawgroup.com/child-custody-and-visitation/.  Primary custody is now defined as more than 60% of custodial time -- see the Rivero opinion posted on the same page.