As a male, it's happened to me quite often, since my age was in single-digits. I just have a naturally serious expression, I guess. It also applies to people fr

As a male, it's happened to me quite often, since my age was in single-digits. I just have a naturally serious expression, I guess. It also applies to people from other cultures like eastern Europe, where it's not the default to go around smiling and laughing in public or making small talk to strangers. Default expressions vary quite a bit and maybe yours is just more cheery or neutral.