Can a 13 yo be a living kidney donor?

No - must be over 18. The decision to donate an organ(kidney), or part of an organ (liver, lung) while living is so important, with consequences for the rest of that person's life, "informed consent" is required. This is only possible for mature persons who are adults and are at least 18 years old. Rarely, because of unique benefit to the twin donor too, courts have allowed pediatric identical twins to be live donors.
Rarely if ever. The age of legal consent in the United States is age 18. However, one may speculate an unusual life saving circumstance when parents may intervene on such an unusual decision. This would require precedent and judicial intervention.