I think I remember seeing 19 somewhere (was it the manual? In game info? I forget), and just shaking my head, lol. The Anniversairy site nor the SNK Wiki say a default age, so I'm not sure where last I saw a number. They just mention she's highschool age, but looks like an elementary school girl.
And noooo, not the Cameo reasoning again!
Of course, Kasumi is ALSO a Cameo in te Kyokugen stage... soooo... (Along with Eiji, Malin, Kushnood, and Hayate.)