Because male and female ideals of beauty are driven by culture and simple genetic imperatives.
I suspect that culture has more influence than instinct or genetics, but cutlure itself may be driven by baser imperatives.
So at a super dumb, reductive, wholly unscientific level - young, nubile (that word literlly means ready or prepared for marriage) and healthy women are perceived both by (most) cultures and possibly by simple animal instincts, as more suitable for safe and successive childbirtth than older women.
Since a male of this species can continue to breed safely well into middle and even old age, the genetic imperatives are probably different too. So an older man who's healthy and fit can be perceived as a suitable breeder. And if he's wealthy to boot, then a good provider. And we see that in society too - older men with wealth and power get to define ideals by reinforcing qualities, or messaging that those qualities are desirable, they themselves possess.
But the above is ONLY meaningful if you think of attractiveness and beauty as mere genetic achievements. I think it's much more complicated than that, and in this culture, with this level of medicine and stability, you're already seeing ideals for beauty in women moving in the right (and sometimes wrong) directions - not just age, but other aesthetic aspects - weight, height, skin color, etc etc etc.
One super ugly aspect of cultural definitions of beauty is that the western fashion industry has developed a bit of a monopoly on beauty ideals, and so you're literally seeing a small group of (unfortunately usually men, but women - Anna Wintour, tons of designers too) people defining a frightening and unhelathy ideal based largely on ridiculous silhouettes that clothes look good on, and an unhealthy drive for youth above all else.
And of course we see evolutions of this in art. No coincidence that reinnaissance ideals of beauty define a kind of robust healthy look above other aspects. They still skew young though, whichgoes back to my original premise.
Kind of a cheat, since the movie was ABOUT that in some ways and leaned into it.