According to perception and this study, people who smile are seen as about 2 years older than they really are. They say that when people smile, “crow’s feet,” the little wrinkles that are at the corners of everyone’s eyes, are more apparent, and that gives the appearance of aging. The study also found that, in order to appear younger, you should walk around with a surprised look on your face all the time. Apparently that’s the key to smoothing out those wrinkles, which gives skin a younger look.
There was an interesting phenomenon in the study though. Long story short, people were shown pictures of three expressions – one smiling, one looking surprised, and one with a neutral look. A majority of participants identified the smiling person as appearing the oldest. However, when recalling the picture after the fact once the study was done, a majority of people recalled the smiling person as being the youngest. A weird, subconscious thing, and alludes to the fact that smiling and happiness is perceived as a “younger” trait.
Smiling people are also perceived as more authentic, spontaneous, and funnier, says another study related to this one.