] Why does Elinor think Edward's "youthful infatuation of nineteen" quite understandable, while at nineteen herself she is quite grown-up and capable of making sensible decisions?

...because he's a man??? :o) I guess I do. Ok, serious answer now.

Elinor knows the kind of man Edward is today, his seriousness of purpose, his intelligence, etc. She also knows the kind of woman Lucy is, "illiterate, artful and selfish." Edward is her superior in every way (except perhaps cleverness!) and would not have become engaged to her if he met her today.