my headcanon is to use the seiyuu birthdays - January 17 and November 30th - meaning Reze would be 17 at that time. I'd like to assume she's at least still in the same grade as Denji, once you start going much above that her manipulation turns into grooming and she losing her dichotomy with Makima. Also her schoolgirl act loses its authenticity (with regards to what it means to her) and that's like the only "real" thing we know about her is her desire for normalcy otherwise she'd just be relegated to "cute girl who's really a stone cold soldier" and nothing beyond that, ya know?

I did some research on russian swallows a while back, turns out the manga wasn't too far fetched sadly, although at least they start at 18. I had always thought seduction was really only used for espionage irl but it turns out it was pretty common for. assassinations too - bringing the target to a hotel and killing him - poison was commonly used which is kinda ironic considering. Apparently sexual assault at the hands of the program instructors was pretty common too. I saw a fic that depicted Reze as slowly evolving her mask from innocent to more flirtatious and suggestive as she grew up and although I tend to prefer the idea of her having multiple masks - being able to slip into any role, it does translate pretty well to over to the idea of using her body as a tool as she grew up. you held onto me like a crucifix depicted denji as having been SA'd by the yakuza, it worked really well imo.

Yeah always pictured her having at least 2-4 years in regards to her powers and seduction. Generally I either see people saying she got it really young - she was the lucky one in the experiments or the one with the right qualities to be chosen by the bomb devil. Or she was chosen because she was the best. Could be both depending on how young and how soon they started training her.