Was Diane Lane really 18 when this was shot?

IMDB lists her birthdate as 22 January, 1965 - which would make her around 17-18 when this was shot (if the movie came out in 1984). So 1) is that birthdate right? Was she really that young? 2) Why would coppola cast an 18 year old in that part? Other than the fact he's made several movies with her.

I think her performance was fine and all, but it just seems weird to cast someone so young in a role that's meant to be older (whereas nowadays you have 20 or 30 something year-olds playing high-schoolers in movie)