It is important to my husband and myself that our children be 18 when they leave for college. I have seen enough 17 year olds who went off to college have complete and utter nightmare first years....several who either dropped out, had emotional issues, or had to move to much smaller schools. Of course, this can happen to anyone, but in OUR EXPERIENCE, it has always been the "younger" freshman.
Now, of course I care about my second grader. However, a bored second grader ( and mine is already 8) can easily be challenged and supplemented at home. In the OP's situation, the "older" child has very little bearing on the academic achievement of her own "young" child. She is just irritated b/c she doesn't want to hear how tall he is.
And for us, I am very, very thankful that my Sept. and Nov. birthday kids are the generally the oldest. We move every 2-3 years and in each state, the cutoffs are much different. If we had started my 7th grader when he was 4 and made the Dec. 1st cutoff (bday Nov. 18) he would now be the way youngest by over a year b/c the cutoff here is Sept. 1st.
He would have been surrounded by much older children in Middle School. Not a situation I would have been thrilled with.