Steve Carrell (53) and Kristen Stewart (26), kind of ridiculous except in Woody Allen's reality. I guess it's an improvement over Bruce Willis (61), who was originally cast in this role. In Woody's last movie he had Emma Stone with Colin Firth. It was super awkward and not believable at all.
I think he uses it more as an interesting recurring topos, so to say. The old-man-young-woman-constellation is very common in movies and literature. Just look at Nabokov's/Kubrick's Lolita, lots of Philip Roth's novels, the latest Michel Houellebecq novel and so forth.
It's just one of those things to write movies/books about.
Of course it might seem a bit intellectually sleazy, but hey...
YOUR VIEWS ARE EXTREMELY AGEIST!! Don't you know ageism is worse than racism or sexism!! Only in USA are people SO ageist!! I have dated many women in Europe & Russia 25-30 years younger than myself!! What is 'creepy" is bigoted people who care that younger women have the intelligence to prefer older men!!
It's possible you're too young to know that in the "golden era" of Hollywood leading men were regularly cast against much younger girls as the main romantic interest. This was the universal norm and no one batted an eyelid about it, especially the legions of young female fans of men like Cary Grant, Tyrone Power, Clark Gable, Gregory Peck, Errol Flynn, Marlon Brando, etc, etc, etc. It's only in recent years the question of age gap has become an issue in Hollywood/TV, and I believe it is a false one.
Out in the real world there are more than enough examples of older men being with younger women, and it is not even uncommon for the younger woman to initiate the relationship. Reality is more complex and unusual than TV or films where people of the same age groups are automatically grouped together because it is the more usual norm. Personally, relationships where the man is much older are not serious and not meant to last, but to say they do not exist and are not much more common than you seem to think would be a mistake. Young women generally don't talk about them, but these "liasons" do happen.
Even today, the biggest male actors are guys in their 40s and 50s, and they are regularly cast against quite younger women. It is how it is. The reasons go beyond the film and television industries and into the nature of men and women.
However, I agree, what Woody Allen did with his step-daughter was morally ambiguous, at best.