(Take this with a grain of salt given that I'm only parenting a 2.8yo and may have no appreciation of the nuances of your DD's age beyond my own experience as a child.)
Could you take the discussion completely out of the body context with her and discuss what it means from a cognitive, emotional, and social perspective to be an adult? That is, discuss how you and your DH changed mentally over time, identify the pluses and minuses of being each age, etc. I have a feeling the underlying issue is one of loneliness and idealizing the perceived ease of finding true friends as an adult. It can be so isolating to have a mature mind "trapped" in a child's body, and to have your thoughts and ideas discounted by outsiders because you're "just a child". That was my superficial take from your OP factum based on my own childhood.
