Clara Bow was an actress in the 1920s who was labeled “The It Girl.” She was the predecessor to Farrah Fawcett, Marilyn Monroe, Raquel Welch and a number of sex symbols who were on TV and movie screens as well as posters on many men’s collegiate dorm walls. Clark Gable, Paul Newman and Brad Pitt might be characterized as having “it” by women.
What is “it?” “It” is a mixture of attractiveness, charm, personality, sex appeal and confidence that radiates from the person. I don’t think that “it” is only a phenomenon or brand for just Hollywood actresses or celebrities. I think that all of us have “it”. “It” are the qualities that make us attractive and what attracts us to a potential spouse, partner and opposite sex. It may also be defined differently as to its constituent qualities.
“It” is not inexhaustible and some people are blessed with more of “It” than others. There are peaks and valleys of “It”. Some people run out of “It” faster than others. Some lucky people never lose it no matter how old they get.
Here is my unscientific, totally subjective and undocumented (no data) analysis of the peaks and valleys when most men and women exhibit IT. Apologies to George Clooney, Morgan Fairchild and Christie Brinkley in advance…

Women acquire “it” earlier than men as they mature more quickly. Women may also lose “it” at a faster rate than men from age range 40-70. The peak age range for both men and woman is from 20 to 30. Again these are generalizations. Depending on one’s heredity, health, financial status, plastic surgery and desire to maintain “It”, results can vary.