Youth has it's allure. That's Nature.
Women tend to like men with broad shoulders, who are taller, narrow waist/pelvis ("V-shape"), moderate muscled, with a full head of non-gray hair. Biology says that is a suitable potential father of children.
Men tend to like women who are shorter, with a low waist-to-hip ratio ("hour glass"), a plump bottom, and moderate breasts. That says she is healthy enough for child-bearing.
That is not to say that other body types, ages, signs of maturity, orientations, etc, are not signals to some people. They are. What I listed are broad generalizations from evolutionary biologists and psychologists.
I consider 25 years old to be "youthful."