So...a female praying mantis?
Jokes aside, canonically, the 3 younger girls in the game, Kateryna, Ain and Nadia are all 3 of marrying age, so somewhere between 18 and 19.
Shani is in her early to mid twenties. 19 at the youngest. Yes, in the setting, marrying age could've been younger than that, but we don't go there both for legal reasons and because we're not interested in developing or delving into the loli fetish, which in and of itself is an open door to understandable cries of pedophilia (I personally would agree that that would be the case, because if you open the door to the girls canonically being 16, there's always going to be that guy who wants her to be 12 or 14) and we have no interest of becoming arbiters or even being involved into the whole fantasy vs reality debate.
What we are more interested into developing anyway, are the characters.
To that point, Ain being a little more sheltered, housebound and possibly a bit spoiled, may come off as being a bit more immature and bratty, which plays a part also into her sexuality. In other words, she's 18 going on 16... Being bratty in the BDSM sense of the term comes more natural to her than to the others, which in turn filters back into her overall personality... But she is in fact an adult.
That said, if you want to shift the age bracket a couple of years in either direction to satisfy your own headcanon or even kink, that's your prerogative. The game will never explicitly state you're wrong... So it's really up to you and what you're comfortable with/your conscience tells you.
The fact that more brutal choices can be made in the game shouldn't mean we endorse any of the content as valid behaviours IRL, not even the softer options, much like for those old enough to remember, the game carmageddon didn't endorse mowing down pedestrians with your car, even if it was a legit game mechanic to beat the level you were playing.