It's widely acknowledged that the Japanese not only tend to look younger than people in the West, some think and behave that way too. After all, this is a nation fostered on kodomo bunka (kiddie culture), visible in everything from fashion to architecture.
For many years, kodomorashisa (childlike-ness) was considered a very good thing, and the highest praise for a man (regardless of age) was shonen-poi (like a little boy) and mujyaki (guileless). For women of course, itsumademo shojyo no yo (forever like a girl) was a virtue that far exceeded mere beauty or intelligence, and the yonareta onna (woman familiar with the ways of the world) was shunned as having lost her mukuna kanji (air of innocence). Maturity was for ok for wine, antiques and jeans -- but the full-fledged Japanese adult was often made to feel taikutsu (boring), fukou (unhappy) and debu (fat).
In the past couple of years however, otona (adult) has become a sweeping marketing concept, especially among women. Otonappoi (adult-like), that once disdained phrase now heads the list of otokokara kikitai kotoba (words we want to hear from men) and the in-trend make-up technique has shifted from shojyogao (little girl face) to otonagao (adult face).