When I was an actor ageism in casting infuriated me.
Director would be considering someone for a role from meeting and seeing and hearing and auditioning him, in other words, IRL, but then find out from words, spoken or on paper, in other words NOT what the actor presents IRL, the actor's actual chronological age and presto - the actor is "too old" or "too young".
This of course happens much more to women than to men.
There is also the ongoing, (and probably dating from the time of Thespis,) tendency of those in charge, (usually male,) to think that a 25 year old woman is just about right to play opposite a 45 year old man.
Well, in some circles, I'll grant, trophy wives or availably anxious, (or anxiously available,) starlets make this plausible.
But when a couple are supposed to be, as inherent in the script, playing contemporaries?
Once Upon a Time in America is one ludicrous example that springs to mind. Or how about Billy Crystal and Meg Ryan as newly-minted college grads? (Although I suppose he might have been getting his Masters or something, thereby explaining the, what? 15 year or so? gap.
Tomorrow on the television program NCIS there are flashbacks to the youth of the character played by David McCallum.
Playing opposite him is Alice Krige.
The actors cast as their younger selves both look thirtyish, and are described in PR for the episode as "childhood friends."
(That said, I'm always delighted to see Alice Krige, and so is Himself, but I suspect his entusiasmt harks back to her Borg Queen persona...)