There is a simpler explanation.
Those older men are known actors/stars
who have established some influence in the industry.
That or some people might buy tickets to watch movies because it has a familiar name in leading role.
The combination of the two above might be enough to explain why the older men can still play lead roles in movies.
Well, that leads to the question, why only men were famous actors past 40, which is where the arguments becomes tautological.
Because men can have attractive roles past 40 and women not, or only very rarely. I don't remember the name of the movie, but it was about greek and a female character that was 40, but the cast was not offered to a 40 year old, but a 30 year old.
Because 40 year old women are not supposed to look like 40 year olds, which is where we are back to the fantasy thing.
Biology is a real thing, though, men can become fathers with over 50, but women cannot. So I am not judging btw. But those fantasies are real and strong, even though they are taboo nowdays.
Biased age disparity in marriage is hardly a "fantasy" or even "taboo". I suspect if you looked at e.g. the top quintile of male earners, or those who married more than once, you're liable to see an even more pronounced gap over the aggregate median. Equally as interesting, how do wealthy divorced/widowed women marry afterwards?
Poking at US presidents that stood out (i.e. more than 5 years):
This shortlist includes the two oldest (Biden, Trump) and youngest (Kennedy) presidents voted into office. A relevant quote attributed to Jacqueline Kennedy[1], whose second marriage was to a businessman 23 years her senior:
> The first time you marry for love, the second for money, and the third for companionship.
Well yes, there is this image, that the successful older men, has a younger wife (and even younger affairs). But the average men has not. Hence serving the "fantasy" of the target audience.
But as far as I observe, this is frowned upon today. It still exists of course, many of the hollywood heros regulary change their 20 year old models for partner, but there is increasing criticism about it. I mean, my partner is 7 years younger and even this was met with criticism.
Criticism from whom? If it's your friends, get better friends. Other than that, it might be family - which is easily explained as jealousy (any success at all tends to get that response). Am I missing some hypothetical case where someone criticises you about the age of your partner and their opinion somehow matters?
> Well, that leads to the question, why only men were famous actors past 40, which is where the arguments becomes tautological.
It was well known that Hollywood had an age discrimination problem for women up until the mid or late 80s or so.
Like a really well known problem where women couldn’t get leading roles after a certain age. It was a well publicized thing and just kind of common knowledge.