Ok I checked for 'nineteen' and found it. Jaak is right: there is a scene when Kris recalls Harey before she died; saying that she was nineteen at the time. I admit I haven't found this scene before.
So she was nineteen. This must be on Gibarian's mind when he says "Twenty". My mistake. But, to conclude things, 19 and 20 is not that much difference.
Either way we can see, that in Sodenbergh's version Harey looks much older than 19. In Tarkovski's movie she's a bit younger, but still looks older than 19. (I'm not telling that I care about what she looks like in the movies, it's just an interesting fact.)
Cheers