Just for fun: How age-(in)appropriate was the Buffy cast?

The Humans
Cordelia: Supposed to be the same age as Buffy. So born in 1980-1981. Charisma Carpenter: 1970. Difference 10-11 years.

Xander: not more than one year older than Buffy (he's seventeen in "Innocence," which takes place on the day after Buffy's 17th birthday). So we'll say 1980. Nicholas Brendon: 1971. Difference: 9 years.

Willow: Supposed to be Buffy's age, so we'll say 1980-1981. Alyson Hannigan: 1974. Difference: 6-7 years.

Oz: Approximately one year older than Buffy, so we'll say 1979-1980. Seth Green: 1974. Difference: 5-6 years.

Riley: as a graduate student, he'd be at least four years older than Buffy.  That would make him born in 1977 at the latest. Marc Blucas: 1972. Difference: Technically anywhere between 2 and 5 years, leaning towards the higher end.

Buffy: canonically born in 1981 (5.22, "The Gift;" the Buffy wiki has somehow pinned it down to January 19 - January I'll accept since the birthday episodes always aired that month). Sarah Michelle Gellar: Born in 1977. Difference: 4 years.

Tara: 1980 (7.04, "Help"). Amber Benson: 1977. Difference: 3 years. 


Faith: Supposed to be Buffy's age, given her ability to waltz in and out of Sunnydale High/UC Sunnydale like she's a student. Eliza Dushku was born in December 1980. Difference: Less than half a year. (For what it's worth, the wiki draws on sources outside the show to give Faith a birth date about fifteen days before Dushku's. Also, edit: I've since come up with a plausible theory that puts Faith's birthday sometime in December 1980. It's possibility #2 in this essay.)

The Supernatural Characters
(Important caveat: I suck* at math. Not as bad as the Mutant Enemy writers apparently do - Spike's supposed to be "barely 200" in his first appearance, but it's later revealed that he was sired in 1880 - but still pretty bad.)
*Oh yes pun intended.

Spike: Born in 1850-1853 (wiki) and sired in 1880 (5.07, "Fool for Love"). Eternally 27-30. James Marsters was born in 1962, making him 30 in 1992, five years before his first appearance. Difference: 5-8 years and counting.

Anya: Eternally 20, according to the wiki. Emma Caulfield: born in 1973. Doesn't appear until 1998, age 25. Difference: 5 years and counting. 

Angel: Born in 1727 (wiki) and sired in 1753 (2.21, "Becoming, Part 1"). That would make him eternally 26. David Boreanaz was born in 1969, which would make him 26 in 1995, two years before the show began. Difference: 2 years and counting.

Dawn (apparent): Is apparently 15 or about to turn 15 in Season 7 (she's starting her freshman year of high school), which began in late 2002. That would make her born in 1987-8. (The wiki says 1986 but doesn't bother to source it.) Michelle Trachtenberg was born in 1985. Difference: 1-3 years. (Of course, Dawn is "actually" either at least eight centuries or only two years old at that point.)