Linguists are still divided on this topic, called the "Critical Period" hypothesis - the question of whether there is a "Critical Period" during childhood when children naturally acquire language better than adults.
The data in favor cited in pop articles often comes from "feral children" like Genie, but as Quetzalcutlass@lemmy.world mentioned, how much of this inability is due to natural brain development and how much is due to years of unimaginable trauma is hard to know.
Other research has cited brain plasticity differences and brain matter changes that occur during puberty that seems like it may be linked to language acquisition.
Again, however, the counterpoint of "It takes ten-ish years of pure immersion for children to learn a language, and how many adults actually do that" is pretty frequent.
I'm still undecided about what I think - maybe something in the middle, like "humans do lose some neuroplasticity during puberty that may inhibit language acquisition a bit, but adults acquiring native-like fluency is still possible with enough immersion".
One of my biggest problems with the movies is that they made Elrond (and all of the other elves) so angry and stodgy, and completely ignored all of the joyful aspects of his character from the books. The elves, especially the Noldor, are a people defined by the way their joy for life is inextricably mixed with sorrow as they fight the long defeat, which is part of what makes them so compelling.