How weird that parents would assume one child was held back a grade. I'd just assume they were born within a year! I suppose that's just my bias since my mother and aunt were, indeed, born within a year of each other (and just barely ended up in different grades).

Mine are 4 years apart in age but the older one so short, and the younger one so tall, that people often assume they're very close in age.

I hope your DD11 gets into the science olympiad if that's what will make everyone's life easier.