Because they take better care of themselves. Also it is a different way of thinking. Nowadays, people in their late 20s and early 30s are young. This age is still the exploratory age. There is no pressure on them to get married and have children. They are free to explore and indulge in various lifestyle options. Our definition of age has changed and evolved from the 20th century to 21st century. Before 1968, people in the late 20s and early 30s were expected to have settled down in terms of marriage and careers. Those who did not settle down were viewed as immature or worse. Presently, such is not the case.
When I was in my late 20s( I am now 61 years of age), I went into a bookstore and into the adult section. The manager informed me that this section was for those over 18 years of age. I told him that I was 28 years of age, he still forbade me from going into the section. If one takes care of oneself, do not smoke, drink, nor take drugs, one can maintain a youthful appearance up throughout life. One also must take scrupulous care of oneself through skin lotions, exfoliators, and skin cleansers. One cannot/should not let oneself go to seed. I never smoked, drank, nor take controlled substances. Also do not eat processed foods. Consume high quality, natural foods.
Also socioeconomics have an influence as to how young is going to look. Maintaining oneself to the premium requires money and lots of it. Higher quality skin care co$t$ money. Higher quality nutrition co$t$ money. People in the upper income brackets no matter what age will look younger than people in the lower income brackets who oftentimes look much older than they are. Money helps people retain their looks, even look younger. With money, one can afford gyms, spas, and other methods of being young.
Then there is the attitude towards getting older. There are those who subscribe, even succumb to the old paradigm of acting. They believe that because they are of a certain age, one must act and dress that age. Then there are those who believe that age is just a number. They view age as evolvement, not the end. They take care of themselves, dress and act youthful. They refuse to permit age from stopping them from being the persons they were meant to be.