Here are the guidelines for Fairfax County. Keep in mind, they are guidelines- you know your child best.
https://www.fairfaxcounty.gov/familyservices/children-youth/child-supervision-guidelines
Age Guidelines
8 years old and younger should always be in the care of a responsible person. Children this age should never be left unsupervised in homes, cars, playgrounds or yards.
9-10 years old may be ready to be left unsupervised up to 1.5 hours during daylight and early evening hours.
11-12 years old may be ready to be left unsupervised up to 3 hours during daylight and early evening hours.
13-15 years old may be ready to be left unsupervised more than three hours but not overnight.
16 years old and older may be ready to be left unsupervised overnight for one to two days, with a plan in place.
Babysitting Age Guidelines
10-12 years old may provide care of other children for up to three hours with the help of an adult.
13-15 years old may babysit infants and children but not overnight.
16 years old and older may watch children overnight.