] 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?

Perhaps Elinor is so used to judging other people by her own standards of maturity and reason (since it seems that for much of her life, she's been the most sensible person she knows) that even Edward must fall short after this revelation. She would be an oddity indeed if she could see her own faults as clearly as other people's, or consider them as serious. A nice bit of characterization here: she is fallible and human, after all!