I think Sayu will be around for a year or two. But Reina could definitely be in until her late 20s. She still looks and acts (sometimes) like a 18 year old and being the ace she is, I could see her be in for quite a few more years. Unless she wants to go and such of course.
Though, I agree. I doubt we'll get a 17 year old, but maybe a girl 15-going-16 will make it at least. A girl with a cool attitude would be neat, no matter the age. Right now we have a bunch of beauty-cuties and a tomboy. Someone cool would be neat. Erina is close, and Riho as well, but so far I'd say they're both falling in the cute category.