When Asuna entered Mahora Academy 7 years ago, she was still a child. Unless she is some magical being who is sexually mature pre-puberty, had a child 10 years ago and forgot about it (assuming even that she did not have to go through 9 months of pregnancy), and finally that Nagi had pedophilia... yes perhaps she could be Negi's mother.
At this point of time, it would seem more likely that Negi travelled back in time to give himself the staff than for Asuna to be Negi's mother... and I absolutely disagree with the former already due to the insane amount of story weaving it would have to involve.
Asuna's age as pointed out earlier cannot be determined by her appearance alone. What we can determine is that ever since she was inducted into Mahora Academy, she had begun to age normally as per normal human beings around her 7 years ago. Prior to that, her age cannot be determined thus she could possibly be as old as a magic itself and that would not matter at all.
Also Negi probably is not so concern about his mother probably because he has definite information about his mother. Perhaps she died, or perhaps he knew her for a while and she's "normal" to him, as oppose to "a great hero who appears when Negi is in danger and happens to wipe out a legion of demons 6 years ago and leaves Negi with a staff and cryptic advice" type of person.