I think I've always looked exactly the age that I am. It's kind of a superpower. When I was 12, I looked 12. When I was 20, I looked 20. Once in a blue moon, someone will think I'm younger than I am, but not often. But guessing someone's age is all about context. When I meet a new person who runs in my circles, I automatically assume that they're my age. Sometimes I'm proven wrong, but I can usually slot people into a certain age group based on the way I meet them. If we're at a concert of the artist we like, or a bar in my neighborhood that I frequent, odds are we have something in common.
But context doesn't always give everything away. Sometimes, when we assume, we make complete fools of ourselves. Like in this recent story from Reddit that feels ripped straight out of a sitcom. Read on and chuckle.