Health magazines and websites try to figure this out for us all the time. Chronological age minus hours of exercise plus cigarettes smoked per week divided by frequency of seat belt use equals how old you really are.
For those of us who read or write books for other age groups, it's much less complicated. Many "YA people" I know have great high school stories that come up in present-day dinner conversations. Their memories of drama club and club drama remain vivid and still mean a lot to them. Somewhere within--and I mean this as a compliment--these people are still somewhere between fourteen and eighteen, and it shows in their reading and writing.
I enjoyed my high school years and remember them fondly, but they're not often on my mind. Elementary school, on the other hand, comes up all the time. I'll use a word and remember how I learned its meaning in first grade (Sarah in the All of a Kind Family books had a rival for the History Prize), or hear a mention of diabetes and remember my fourth grade science fair project on the subject. I remember my first reactions to many favorite middle-grade novels better than my responses to most YA books.
My YA years and my picture book years certainly mattered to me, and a Francisco Stork or Mo Willems book can still blow my mind (as can a Barbara Kingsolver; my actual current age does play a role in my reading choices when it gets a chance). But I suspect that Beverly Cleary, Sara Pennypacker, and I have the most in common: at least part of each of us is about eight years old.
And how old are you?