Yes and no. I had some friends when I was 16/17 who could have passed for being in their mid-twenties… others would have struggled to get in to 15 rated films without ID. If your characters are in their twenties, you might be able to pull it off, but probably not if they're playing balding 48 year olds.
Of course, the acting capabilities and emotional maturity of your actors are equally important and they can only be chosen by you, but no matter how fine their acting is, if you don't pass the first hurdle of appearance and voice you'll never convince the audience.