Honestly I work in healthcare so I see tons of people every day and I get to see their birthdate....most of the Internet people saying that are simply delusional. I've noticed that everyone on the Internet thinks they look incredibly young for their age, but in real life virtually everyone looks their age or close to it.
I have definitely seen some freak outliers, but that's the point, they are rare enough that they stood out amongst thousands and thousands of people. Meanwhile if you look at YouTube video, half the comments are bragging about how they look 20 years younger than their age, which is just asinine.
Maybe it's because people post a lot of filtered pics these days so people comment that they look young, or the person starts to perceive themselves as actually looking like the filtered pic in person? Definitely a weird phenomenon.