As a rule of thumb, people up to age 30 or so have an average maximum 7mm pupil dilation. When you reach 40, on average again, your pupil can only dilate to 5mm.
I said average because I'm 52 and my maximum pupil dilation isn't in the "normal" range. I know of a person on this board who is older than me, and his pupil dilation falls way outside the normal range for his age.
