There is nothing wrong with saying "younger than me", but "younger than I am" is more technically correct, so it ought to be accepted.
Would it be incorrect to say 'hai năm' here?
she is younger than me two year. is this sentence correct?
"Two years younger than" is the usual idiom. If you put the "two years" at the end of the sentence you need a preposition, i.e., "younger by two years." Either way, "years" has to be plural in standard English.
yes, thank you very much
"Younger than I" is correct. The conversational use of "me" does not make standard grammar incorrect.
yes, that would be wrong.