I (F30) have a niece who is 13. Up until a week ago, she still believed in Santa. I asked her parents (my brother and SIL) if she was just pretending to still believe to not make them feel bad, or maybe to get more gifts or something, but they were adamant she still believed.
Last Christmas Eve I was at their house for dinner, and she kept saying things like “I’m so excited for Santa to come tonight” and telling me everything she had asked him for and wondering what country he was in at that very moment. She doesn’t have any younger siblings, so it’s not like she was saying this for anybody else’s benefit.
She is a smart girl, does well in school, and has no developmental delays that I know of. She just, for whatever reason, still believed in Santa at age 13.