@Emma@WeepingLiberty @Ellion Correct me if I get any of this wrong. It's your RP, after all.
Going backwards:
A 20 year old Gem who's married would definitely still be eligible for Reaping.
We never ironed out exactly how Reaping works, but I always pictured a situation where at a certain time of year, all Gem girls of 18 or over (with probably an upper age limit of like 30 or something even though being chosen after like, 24 would be a serious anomaly) are required to gather and be picked over by the Drakkan Reapers. They wouldn't give two fucks about which family they came from or whatever, and would probably just take whoever they thought would please the Lords the most (probably limited to a certain number of gems from each village, maybe dependent on population, but still have a certain number they need to bring back in total).
For example, my character Lienna was the only Gem reaped from her village because of the small population, and the fact that girls with the potential to be beautiful were drowned as babies (a fate thought kinder than the potential of Reaping).
I can't imagine the Drakken letting the Gem leaders choose who gets Reaped. They'd probably be suspicious that the gems were offering less-than-ideal girls and would want to choose them by hand.
That said, I don't see the Reapers refusing a volunteer or a girl who was handed over by the other gems, assuming she wasn't ugly or known to be barren or really old or something.
The Reaped gems are then escorted back to Shadow Worth (are we still using Shadow Worth?)/Drakka in carriages, fed, kept safe and roughed up as little as possible on the way. Can't be bringing damaged goods to the choosing ceremony, after all.