Just when trying to guess someone’s age (we’ll assume completely family-friendly and above board), think back to high school. How old did you and your peers look? Now go take a look at high schoolers today. They probably seem a lot younger than you did. The longer it’s been (i.e. the older you are), the younger they look. Which means, “when I see it” depends entirely on the age of the viewer.
This isn’t even just about perception and memory- modern style is based on/influenced heavily by youth. It’s also continuing to move in the direction. This is why actors in their 30s - with carefully managed hair, skin, makeup, and wardrobe - have been able to convincingly portray high schoolers. This means that it’s not just you - teens really are looking younger each year. But they’re still the same age.