For body type, 10 is peak for a loli vamp. It could up to 12 but no more.
Now, for real years the issue becomes more tricky. For me it would be no less than 300. By that time she probably already got used to her vampirism and is not angsty anymore about not growing up. For example, Claudia from Interview was still in that period of angst which made her act like a cunt at times. If she survived her personality would have probably changed for the better.