How old is Dawn, really, relative to the others? It's always possible she's really on a sliding timeline and the Key is constantly rewriting when she was retroactively born... but still. I thought the difference was like six years.
If every season is definitely a year of plot-time, then it's a little weird:
Season 5: Buffy turns 20. Dawn's age: ??? Season 6: Dawn's 15 as of Buffy: The Musical. Season 7: Dawn starts high school, making her still at most 15; Buffy's 22.