-Genetics cannot be applied here. Sure, they *look* human, but they have magic, blue hair, dragon/fox/wolf genes, etc. If they were humans with 9 month gestation periods, you could effectively pass years between babies. If Elise is 13, but Silas has a baby with Mozu, she's 13.75 after Sophie is born. Do that paralogue, get Sophie. Then Niles gets hooked up. Do that paralogue, get NIna. Elise is now 14.5. Camilla and Benny share a romantic dinner, Ignatius happens. Recruit Ignatius. Elise is 15.25. Laslow and someone male Soleil. Elise is 16. Lather rinse repeat 4 more times, and Elise is 19. Does she look 19? Certainly not by the standards we're used to.
-Nyx certainly doesn't *act* like she's been around a hundred or so years. Her attitude strongly reminds me of when I was in third grade, and the girls in my class would run around and say to the boys, "Girls mature faster than boooooyyyyysss~", completely oblivious to the irony that entailed. Every time you talk to her, she's touting it over her head like she's got something to prove. WHO YOU TRYIN' TO CONVINCE, NYX? YOU AIN'T FOOLIN' OLD MINT!
But for the sake of the story, I'll buy that she's a powerful witch that cursed herself with an eternally prepubescent body. Given her attitude, she obviously hasn't been around long enough to see real people grow old and die, so certainly less than four generations old. If she was venerable when she cast the spell (it's usually old wizards who fear death that end up becoming liches), we'll say she was 60-70 when she cast it, and maybe another 1-5 years have passed in solitude before meeting the party, since she's still super awkward when meeting people in her child-form. Even a small amount of exposure to strangers would give you the chance to get used to social interaction as an immortal child. I mean.. You need milk and toilet paper, don't you? You might want to go swimming in the summer with your newfound youth?
61-75 years old, for the sake of the argument.
Now, realistically, the whole "OMG I'M ACTUALLY OLDER THAN YOU WTFFFF RESPECT ME!" is a totally rebellious-teen attitude. She's not actually a super powerful old witch. She's just a kid dabbling in the dark arts (Look at her stats when you get her. Is that the sort of person you imagine when you see someone that can cast a spell of immortality? No. Stop lying. It's not). Her parents got mad at her, she took off and is living on the streets. She got tired of begging for change, and concocted this story. "I have no family" is right, but not for the reason she's telling you.
13-16 years old. Give up the lies, Nyx. Your family still loves you, no matter how your last conversation ended!