It is a widely acknowledged fact in Hollywood today that leading men are permitted to age on screen while their female colleagues are required to keep at least one toe dipped in the fountain of youth. The charts in this Vulture article compare the ages of leading men and their love interests film by film. Though actors like Denzel Washington, Harrison Ford, Johnny Depp, Tom Cruise, George Clooney, and Richard Gere age into their 50s and 60s, only seldom are their leading ladies as old as 40.
This problem isn't unique to the film world. A recent British study shows that older female television presenters are pushed off screen.
"...out of 481 presenters at all the networks only 26 women over 50 are regularly on screen. Of presenters over 50, just 18% are women. This group makes up just 5% of presenters of all ages and sexes and 7% of the total workforce both on and off screen. While there are regularly 188 women on screen, making up 39% of that workforce, the majority of them are under 50." (Women and Hollywood)