^ my preference would definitely be for an older, more mature Supergirl. Probably in her early to mid 20s, rather than a highschool student. Superheros (male and female) always seem to have love interests in these films. It would be nice to see someone who would be a good match for her from the comics, like Brainiac 5.
I think 18 years old is old enough but not too old to call her Supergirl. if she is in her mid twenties, she should probably be called Superwoman. A girl implies a juvenile, probably early college age, so as not to get in trouble with dating and so forth. The thing about supergirl is that the actress playing her will grow up fast if it is live action, so when they decide to produce a live action movie will determine which actress they select.