Reading your comment about the second clip being many years later, I sort of expected being able to pick up time showing in her body and movements. I am a rank amateur but I could not detect any deterioration. Granted, a different ballet, but still...
The fact that I am considered an adult is both terrifying and hillarious
I don't know much about it either, but I like watching it - have done so all my life.
No, prima ballerinas can go on well into their fifties - if anything, they improve with age. The first clip is of course much more athletic than the second one, but I have no doubt that she would've had no problem with performing it at this later stage.
What was really noticeable for me in the first clip was her maturity, given her age, and then the very noticeable way she fixes her orientation every time before she goes into the spinning routines. (They call it 'spotting', they need to do it to keep track of where they are on stage, and to whip their heads around at the end of every turn so that they don't develop vertigo). I'm sure that you wouldn't have been able to see it in the later Darcey.