Sexual relations between adults and children are illegal; it is not ‘just a number’. Spend enough time around abused children and the devestating ramifications of such ‘relationships’ and the reasons for such laws become painfully clear. 17 is still under age, it is still against the law regardless of how you feel about this particular issue. It is not ok simply because they are ‘in love’, it is an adult taking sexual advantage of an underaged minor.
If you like the movie for its art or its coming of age themes etc, go for it but that doesn’t change the legality of the relationship, which even if it was consensual, it still statutory rape.