You could look at it as sad.
I do like your take on it as a metaphorical thing, i.e. she'll never be 11 years old again, and you can never meet the same person twice. Very perceptive - you can never cross the same river twice, that is certainly true. And even as the world changes others, I am changed, too...
But no - it's not sad. I meet a lot of people, all the time, and a lot of them I like a lot, but I know that I'll never see them again. That isn't a sadness, though; the sadness would be if I never got to meet them in the first place.
I take a lot from meeting people. It makes me feel very happy, because even if I've only known them for an hour of two, I've still known them, and they will always stay with me.
So, I'm not sad.