[@Emma] the way it's worked before has been that a party of drakkans will go to the Gem towns and grab the pretty girls, bringing them to the Fortress on the border of Gemminia and Drakka so that the lords and whoever could just go there and pick the ones they wanted without having to go through the "dragging screaming girls away from home" bit themselves. as for the Age range, they need the Gems to be able to have kids for them, so younger ones are preferred. I suppose they might grab an older 20's woman if she were pretty enough, but It's unlikely. 

The Gems don't really gather for the reaping, they actually try to hide their kids away from the Drakkans when they show up. I remember there being something about Gems even killing beautiful girls on purpose so they wouldn't be taken.