Can a 20 year old spend the night with someone without their parents permission?

Yes, a 20-year-old can spend the night with someone without their parents' permission, as they are legally considered an adult in most places. At this age, individuals are generally able to make their own decisions regarding personal relationships and living arrangements. However, it's always a good idea for young adults to communicate openly with their parents about their plans, even if permission isn't legally required.