i think all the personalities are different ages. i've always thought of it as a family, almost:
father: cranky, middle-aged or older
mother: snooty, middle-aged or older
oldest sister: normal, college-aged (20's)
oldest brother: smug, college-aged (20's)
middle sister: uchi, teenager
middle brother: jock, teenager
youngest brother: lazy, little kid (maybe 9 or 10)
youngest sister: peppy (maybe 6 or 7)
that's not to say that any of them are actually related (besides chrissy/francine etc), but that's the model i always see the villagers most resembling. :]